PHP is a server-side scripting language, that is used to develop Static websites or Dynamic websites or Web applications. PHP stands for Hypertext Pre-processor, that earlier stood for Personal Home Pages.

PHP scripts can only be interpreted on a server that has PHP installed. The client computers accessing the PHP scripts require a web browser only. A PHP file contains PHP tags and ends with the extension “.php”.

PHP is an open-source server-side scripting language that many devs use for web development. Easy to learn: PHP is not hard to learn for absolute beginners. PHP has a very supportive online community.

Variables	Constants
The value of a variable can be changed during the execution.	The constant value can’t be changed during script execution.
Variables require compulsory usage of the $ sign at the start.	No dollar sign ($) is required before using a constant.
It is possible to define a variable by simple assignment.	Constants can’t be defined by simple assignments. They are defined using the define() function.
The default scope is the current access scope.	Constants can be accessed throughout without any scoping rules.

PHP Engine creates a logical object to preserve data across subsequent HTTP requests, which is known as session.

Sessions generally store temporary data to allow multiple PHP pages to offer a complete functional transaction for the same user.

Simply, it maintains data of an user (browser).

PEAR means “PHP Extension and Application Repository”. It extends PHP and provides a higher level of programming for web developers.
 

PEAR provides a higher level of programming for web developers. It contains all kinds of PHP code snippets and libraries. It also provides you with a command-line interface to automatically install packages.

The old name of PHP was Personal Home Page.

PHP can be considered as a partial case-sensitive language. The variable names are completely case-sensitive but function names are not. Also, user-defined functions are not case-sensitive but the rest of the language is case-sensitive.

Ex:
User Defined functions in PHP can be defined in lowercase but later referred to in uppercase, and it would still function normally.

In static websites, content can't be changed after running the script. You can't change anything on the site. It is predefined.

In dynamic websites, content of script can be changed at the run time. Its content is regenerated every time a user visit or reload. Google, yahoo and every search engine is the example of dynamic website.

PHP4 doesn't support oops concept and uses Zend Engine 1.

PHP5 supports oops concept and uses Zend Engine 2.

Just use the PHP command line interface (CLI) and specify the file name of the script to be executed.

• WordPress
• Joomla
• Magento
• Drupal

There are 8 primary data types in PHP.

Integers: Integers are whole numbers without a floating-point. Ex: 1505.

Doubles: Doubles are floating-point numbers. Ex: 17.540

Booleans: It represents two logical states- true or false.

NULL: NULL is a special type that only has one value, NULL.

Arrays: Array is a named and ordered collection of similar type of data. Ex: $fruits = array("mango", "orange", "banana")

Strings: Strings are a sequence of characters. Ex: 'Hello World!'

Objects: An instance of classes containing data and functions. Ex: $orange = new Fruit();

Resources: Resources are special variables that consist of references to resources external to PHP Ex: database connections.

To be able to display the output directly to the browser, we have to use the special tags <?= and ?>.

• CakePHP
• CodeIgniter
• Yii 2
• Symfony
• Zend Framework

PHP supports only single inheritance, it means that a class can be extended from only one single class using the keyword 'extended'.

• A variable must start with a dollar symbol, followed by the variable name. Ex: $price = 200; where price is a variable name.
• Variable names must begin with a letter or underscore.
• A variable name can consist of letters, numbers, or underscores. But you cannot use characters like + , – , % , & etc.
• A PHP variable name cannot contain spaces.
• PHP variables are case-sensitive. So $NAME and $name both are treated as different variables

Echo	Print
echo can output one or more strings.	print can only output one string and it always returns 1.
echo is faster than print because it does not return any value.	print is slower compared to echo.
If you want to pass more than one parameter to echo, a parenthesis should be used.	Use of parenthesis is not required with the argument list.

'final' is introduced in PHP5. Final class means that this class cannot be extended and a final method cannot be overridden.

PHP echo output one or more string. It is a language construct not a function. So the use of parentheses is not required. But if you want to pass more than one parameter to echo, the use of parentheses is required.

PHP print output a string. It is a language construct not a function. So the use of parentheses is not required with the argument list. Unlike echo, it always returns 1.

In PHP, @ is used for suppressing error messages. If any runtime error occurs on the line which consists @ symbol at the beginning, then the error will be handled by PHP.

$message stores variable data while $$message is used to store variable of variables.

$message stores fixed data whereas the data stored in $$message may be changed dynamically.

PHP is a server-side language whereas HTML is a client-side language. So PHP executes on the server-side and gets its results as strings, objects, arrays, and then we use them to display its values in HTML.

A PHP parser is software that converts source code into the code that computer can understand. This means whatever set of instructions we give in the form of PHP code is converted into a machine-readable format by the parser.

PHP and Javascript cannot directly interact since PHP is a server side language and Javascript is a client-side language.

However, we can exchange variables since PHP can generate Javascript code to be executed by the browser and it is possible to pass specific variables back to PHP via the URL.

imagetypes() gives the image format and types supported by the current version of GD-PHP.

PHP constants are name or identifier that can't be changed during execution of the script. PHP constants are defined in two ways.

• define()
• const()

Indexed Array
An array with a numeric key is known as the indexed array. Values are stored and accessed in linear order.

Associative Array
An array with strings for indexing elements is known as the associative array. Element values are stored in association with key values rather than in strict linear index order.

Multidimensional Array
An array containing one or more arrays within itself is known as a multidimensional array. The values are accessed using multiple indices.

PHP magic constants are predefined constants, which change based on their use. They start with a double underscore (__) and end with a double underscore (__).

The functions are getimagesize() for size, imagesx() for width and imagesy() for height.

The 4 main types of errors in PHP are:

Notices
Notices are non-critical errors that can occur during the execution of the script. These are not visible to users.
Ex: Accessing an undefined variable.

Warnings
These are more critical than notices. Warnings don’t interrupt the script execution. By default, these are visible to the user.
Ex: include() a file that doesn’t exist.

Fatal
This is the most critical error type which, when occurs, immediately terminates the execution of the script.
Ex: Accessing a property of a non-existent object or require() a non-existent file

Parse Error (Syntax)
Parse errors are caused by misused or missing symbols in a syntax. The compiler catches the error and terminates the script.
Ex: Unclosed brackets or quotes, Missing or extra semicolons or parentheses, Misspellings

Traits are a mechanism that lets you create reusable code in PHP and similar languages where multiple inheritances are not supported. It’s not possible to instantiate it on its own.

A trait is intended to reduce the limitations of single inheritance by enabling a developer to reuse sets of methods freely in many independent classes living in different hierarchies of class.

• for
• while
• do-while
• for each

If the function require() cannot access the file then it ends with a fatal error. However, the include() function gives a warning, and the PHP script continues to execute.

require(), and require_once() perform the same task except that the second function checks if the PHP script is already included or not before executing it.

The crypt() function is used for this functionality as it provides a large number of hashing algorithms that can be used. These algorithms include sha1, sha256, or md5 which are designed to be very fast and efficient.

The PHP count() function is used to count total elements in the array, or something an object.

The set_time_limit(0) added at the beginning of a script sets to infinite the time of execution to not have the PHP error ‘maximum execution time exceeded.’

It is also possible to specify this in the php.ini file.

A cookie is a small record that the server installs on the client’s computer. They store data about a user on the browser. It is used to identify a user and is embedded on the user’s computer when they request a particular page. Each time a similar PC asks for a page with a program, it will send the cookie as well.

After verifying the user’s identity in encrypted form, cookies maintain the session id generated at the back end. It must reside in the browser of the machine. You can store only string values not object because you cannot access any object across the website or web apps.

A cookie is created with the setcookie() function.

Note: The setcookie() function must appear BEFORE the <html> tag.

The header() function is used to send a raw HTTP header to a client.

It must be called before sending the actual output.

This is a PHP syntax error expressing that a mistake at the line x stops parsing and executing the program.

ASP.NET	PHP
A web application framework.	A server-side scripting language.
It is designed for use on Windows.	It is Platform independent
Code is compiled and executed.	Interpreted mode of execution.
It has a license cost associated with it.	PHP is open-source and freely available.

file_get_contents() lets reading a file and storing it in a string variable.

The isset() function checks if the variable is defined and not null.

We must use mysqli_connect() function as below

The session_start() function is used to start a new session. Also, it can resume an existing session if it is stopped. In this particular case, the return will be the current session if resumed.

The session_destroy() function is used to destroy all of the session variables.

An array is used to store multiple values in a single value. In PHP, it orders maps of pairs of keys and values. It saves the collection of the data type.

mysql_pconnect() ensure a persistent connection to the database, it means that the connection does not close when the PHP script ends.

This function is not supported in PHP 7.0 and above

The result set can be handled using

There are many array functions in PHP

• array()
• array_change_key_case()
• array_reverse()
• array_keys()
• count()
• array_search()
• array_map()
• array_merge()
• sort()
• array_chunk()
• array_filter()
• array_intersect()

PDO stands for PHP Data Object. PDO is a set of PHP extensions that provide a core PDO class and database, specific drivers.

The PDO extension can access any database which is written for the PDO driver. There are several PDO drivers available which are used for FreeTDS, Microsoft SQL Server, IBM DB2, Sybase, Oracle Call Interface, Firebird/Interbase 6 and PostgreSQL databases etc.

Earlier versions of PHP used the MySQL extension. However, this extension was deprecated in 2012.

GET	POST
GET method is used for requesting data from a specified resource.	POST is used for sending the data to the server as a package in a separate communication with the processing script.
GET method cannot be used for sending binary data like images or word documents	The POST method can be used to send ASCII as well as binary data like images and word documents
Data is sent in the form of URL parameters which are strings of name-value pairs separated by ampersands(&)	Data sent through the POST method will not be seen in the URL
This method must not be used if you have any sensitive information like a password to be sent to the server.	Sensitive information can be sent using this method.
You can use this method only for data that is not secure.	Data sent through this method is secure.
It can be used for submitting the form where the user can bookmark the result.	Submissions by form with POST cannot be bookmarked.
GET method is not safer since parameters may be stored in web server logs or browser history.	POST method is safer than GET because the parameters are not stored in web server logs or browser history.

The function mysqli_num_rows() returns the number of rows in a result set.

The strlen() function is used to get the length of the string.

In PHP, type hinting is used to specify the expected data type (arrays, objects, interface, etc.) for an argument in a function declaration. It was introduced in PHP 5.

Whenever the function is called, PHP checks if the arguments are of a user-preferred type or not. If the argument is not of the specified type, the run time will display an error and the program will not execute.

You can check other features below.

• makes you code more stable, especially if the project is large
• the code is more readable, cleaner and professional
• gives you autocomplete in your IDE
• you can more easily understand the code and how methods are working when you come to a new project

There are many array functions in PHP

• strtolower()
• strrev()
• strlen()
• lcfirst()
• ucfirst()
• ucwords()
• strtoupper()

The mysqli_fetch_object() function collects the first single matching record where mysqli_fetch_array() collects all matching records from the table in an array.

To terminate the execution of the script in PHP, the exit() function is used. It is a built-in function that outputs a message and then terminates the current script.

You can use JavaScript submit() function to submit the form without explicitly clicking any submit button.

To access the data sent via the GET method, we use $_GET

To access the data sent this way, you use the $_POST array.

• include
• require

It is possible to use the dedicated function, is_numeric() to check whether it is a number or not.

If we want to check whether a variable has a value or not, it is possible to use the empty() function.

The unlink() function simply deletes the file given as entry.

The unset() function a variable undefined.

The stripslashes function remove the escape characters before apostrophes in a string.

The addslashes function escape data before storage into the database.

The strip_tags() function clean a string from the HTML tags.

Using the global keyword.

PHP fwrite() and fputs() functions are used to write data into file.

The unlink() function is used to delete a file in PHP.

A function returns a value using the instruction ‘return $val;’.

The move_uploaded_file() function is used to upload file in PHP.

We use an ampersand in front of it, as follows $var1 = &$var2

You can download file using readfile() function

mail() function is used to send email in PHP.

func_num_args() is used to give the number of parameters passed into a function.

The .htaccess is a configuration file on Apache server. You can change configuration settings using directives in Apache configuration files like .htaccess and httpd.conf.

The PHP explode() function breaks a string into an array.

split() function splits string into an array by regular expression.

$$var2 contains the value 10.

A persistent cookie is permanently stored in a cookie file on the browser's computer.

By default, cookies are temporary and are erased if we close the browser.

:: is used to access static methods that do not require object initialization.

Objects are passed by reference.

• Integer
• Boolean
• Float
• String
• Array
• Object
• Callable
• Null
• Resource

__sleep returns the array of all the variables that need to be saved, while __wakeup retrieves them.

$variable3 will contain “Hello World”. The first code is faster than the second code especially for large large sets of data.

To concatenate two string variables together, we use the dot (.) operator.

The session_unregister() function unregister a global variable from the current session and the session_unset() function frees all session variables.

$GLOBALS is associative array including references to all variables which are currently defined in the global scope of the script.

A lambda function is an anonymous PHP function that can be stored in a variable and passed as an argument to other functions or methods. A closure is a lambda function that is aware of its surrounding context

$_SERVER is an array including information created by the web server such as paths, headers, and script locations.

$_FILES is an associative array composed of items sent to the current script via the HTTP POST method.

$_FILES[‘userfile’][‘error’] contains the error code associated with the uploaded file.

We can change the maximum size of files to be uploaded by changing upload_max_filesize in php.ini.

Exception::getMessage lets us getting the Exception message and Exception::getLine lets us getting the line in which the exception occurred.

The boolean function isset determines if a variable is set and is not NULL.