A PHP script is executed on the server, and the plain HTML result is sent back to the browser.


PHP Syntax:-

A PHP script can be placed anywhere in the document. A PHP script starts with <?php and ends with ?> 


<?php// PHP code goes here?>



The default file extension for PHP files is (.php) . A PHP file normally contains HTML tags, and some PHP scripting code. Below, we have an example of a simple PHP file, with a PHP script that uses a built-in PHP function "echo" to output the text "Hello World!" on a web page: 



Note: PHP statements end with a semicolon (;).


Comments in PHP:-

A comment in PHP code is a line that is not executed as part of the program. Its only purpose is to be read by someone who is looking at the code.  Comments can be used to Remind yourself of what you did - Most programmers have experienced coming back to their own work a year or two later and having to re-figure out what they did. Comments can remind you of what you were thinking when you wrote the code.

PHP supports several ways of commenting Multiple line comments and single line comment.

// This is a single-line comment
# This is also a single-line comment
/*
This is a multiple-lines comment block
that spans over multiple
lines
*/


// You can also use comments to leave out parts of a code line



Case Sensitivity:- 

In PHP, all keywords e.g. if, else, while, echo, classes, functions, and user-defined functions are NOT case-sensitive. 
In the example below, all three echo statements below are legal

<!DOCTYPE html>
<html>
<body>

<?php
ECHO "Hello World!<br>";
echo "Hello World!<br>";
EcHo "Hello World!<br>";
?>


</body>
</html>


However:- all variable names are case-sensitive. In the example below, only the first statement will display the value of the $color variable (this is because $school, $SCHOOL, and $schoOL are treated as three different variables):

<!DOCTYPE html>
<html>
<body>

<?php
$color = "red";
echo "My car is " . $school . "<br>";
echo "My house is " . $SCHOOL . "<br>";
echo "My boat is " . $schoOL . "<br>";
?>


</body>
</html>


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