What is PHP?

PHP (Hypertext Preprocessor) is an open-source server-side scripting language that is widely used for web development. It was created by Rasmus Lerdorf in 1994.



Features of PHP:

• Effective for server-side scripting: PHP is specifically designed for web development and can be embedded into HTML, making it an efficient tool for server-side scripting.
  
  
• Open Source: Being open source, PHP is free and is developed and updated by a community of developers who work to improve it.
  
  
• Cross Platform: PHP runs on various platforms like Windows, Linux, Unix, and many more.
  
  
• Supports a wide range of databases: PHP supports many types of databases, including MySQL and MSSQL, making it a versatile choice for web development.
  
  
• Supports Object-Oriented Programming: PHP supports object-oriented programming, which allows for complex, scalable, and reusable code.
  

Important PHP commands and their uses:

• echo: This command is used to output one or more strings.
  
  
• $...VAR: This is a variable in PHP. It is a container for storing data.
  
  
• if...else...elseif: These are conditional statements used to perform different actions based on different conditions.
  
  

Using PHP in Web Development

In web development, PHP is widely used because it can be embedded within HTML code. This capability makes it very convenient to use and allows for greater flexibility when developing webpages.

PHP Syntax

A PHP script can be placed anywhere in the dоcument and starts with <?php and ends with ?>. A PHP file normally contains HTML, CSS, JavaScript, and PHP code. The default file extension for PHP files is ".php".

Creating a Simple PHP Script

<?php
echo "Hello, World!";
?>

The above script will print the text "Hello, World!" when run on a server using PHP.

PHP Variables

In PHP, a variable starts with the $ sign, followed by the name of the variable. PHP automatically converts the variable to the correct data type, depending on its value.

<?php
$txt = "Hello, World!";
$x = 5;
$y = 10.5;
?>

PHP Functions

A function is a block of statements that can be used repeatedly in a program. A function will not execute immediately when a page loads, but it will be executed by a call to the function.

Below is an example of a simple PHP function:

<?php
function writeMsg() {
  echo "Hello, World!";
}

writeMsg(); // calling the function
?>

In the code above, we define the function named "writeMsg". When the function is called, it will print the string "Hello, World!".

Working with PHP forms

PHP is frequently used to collect form data. HTML forms are a flexible and interactive way to collect user input, and PHP can process these forms to create a dynamic response.

GET and POST

PHP supports various methods of form data submission, but the most common ones are GET and POST.

• GET: This method appends form data to the URL in name/value pairs. The length of a URL is limited (about 3000 characters). Never use GET to send sensitive data! Furthermore, GET is better for non-secure data, like query strings in Google.
  
• POST: This method sends the form data as HTTP body data. POST has no size limitations, and can handle binary data efficiently.
  

PHP and MySQL

PHP combined with MySQL are cross-platform and is an excellent option for web developers around the globe. They are used for developing web-based software applications.

PHP Sessions

A session in PHP is a way to preserve certain data across subsequent accesses. When you're working on an application, you open it, do some changes and then you close it. This is much like a Session. It helps to store user data to be used across multiple pages. When a session is started following things happen -

• PHP first checks if a session is already started or not.
• If a session is already started, PHP retrieves the old session and then uses the same.
• If a session is not already started, PHP starts a new session.
  

Here's how you start a session:
<?php
// Starting a session
session_start();
?>

And an example of setting and accessing PHP session variables:
<?php
// Starting a session
session_start();

// Setting a session variable
$_SESSION["username"] = "John Doe";

// Accessing a session variable
echo 'Hi, ' . $_SESSION["username"];
?>

PHP and MySQL Connectivity

PHP, combined with MySQL, forms a major component of the LAMP stack (Linux, Apache, MySQL, PHP), a very popular open-source web development platform. Let's look at how to establish a basic connection with a MySQL database using PHP:
<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {
  die("Connection failed: " . $conn->connect_error);
}
echo "Connected successfully";
?>

PHP and Cookies

In PHP, a cookie is often used to identify a user. A cookie is a small file that the server embeds on the user's computer. It lets the website remember information about the visitor between visits.

To set a cookie in PHP, you use the setcookie() function.
<?php
  setcookie("user", "John Doe", time()+3600);  // expires in one hour
?>
To retrieve the value of the cookie, you can use the $_COOKIE superglobal array.
<?php
  if(!isset($_COOKIE["user"])) {
    echo "Cookie named 'user' is not set!";
  } else {
    echo "Cookie 'user' is set!<br>";
    echo "Value is: " . $_COOKIE["user"];
  }
?>

Error Handling in PHP

Error handling is important in any scripting language. When your code doesn't go as expected, error handling can help you understand where the problem lies.

In PHP, the die() function can be used to stop the script if an error occurs. This function prints a message and then exits the script.
<?php 
  if(!file_exists("welcome.txt")) {
    die("File not found");
  } else {
    $file=fopen("welcome.txt","r");
  } 
?>

Object-Oriented Programming in PHP

PHP supports object-oriented programming (OOP). Using OOP in PHP can help to organize your code better, make it more reusable, and easier to maintain. The main principles of OOP include classes, objects, inheritance, properties, methods, and much more.

A class is defined almost like a standard PHP function, but the keyword to define it is 'class'. Here's a very basic example of a class:

<?php
class Greeting {
  public function sayHello() {
    echo "Hello!";
  }
}

$greeting = new Greeting();
$greeting->sayHello();
?>

File Handling in PHP

PHP provides a range of functions to handle files and directories. You can open, read, write, delete, and close files; as well as make detailed inquiries about your files directly within your PHP scripts.

Here is a simple example of how to open and read a file in PHP:

<?php
$file = fopen("test.txt", "r") or die("Unable to open file!");
echo fread($file,filesize("test.txt"));
fclose($file);
?>

PHP and AJAX

AJAX (Asynchronous JavaScript and XML) is a technique for creating fast and dynamic web pages, allowing web pages to be updated asynchronously by exchanging small amounts of data with the server.

PHP and AJAX are often used together to load data onto a webpage without needing to refresh the whole webpage. Data can be retrieved from a server, or sent to a server in the background.

Here's a simple illutration of how PHP can respond to an AJAX request:

<?php
if ($_SERVER['REQUEST_METHOD'] == 'GET') {
    $name = $_GET['name'];
    echo "Hello, " . $name;
}
?>

This only scratches the surface of the myriad applications and functions of PHP. There's still many other topics to further explore such as interacting with APIs, using libraries and frameworks, security best practices, and a lot more.