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PHP

PHP runs on the server. Particularly, it can process forms, files and databases over the internet.

To use PHP, you must first rent a server from a web host such as iPage, or set up your own server (Chapter 13). There are all-in-one packages that allow PHP to be run on a local computer, such as XAMPP and WampServer. These distributions contain Apache, PHP, MySQL and other applications.
Usage of Server-Side Languages, 15 Nov 2014
PHP 82.0%
ASP.NET 17.3%
Java 2.7%
ColdFusion 0.7%
Perl 0.6%
Ruby 0.5%
Python 0.2%
JavaScript 0.1%
Erlang 0.1%
Others (<0.1% each):
Lasso, Scala, Tcl, Smalltalk, C++, Haskell, Lisp, Ada
(Courtesy of http://w3techs.com/technologies/overview/programming_language/all)
A manual for PHP, protected by the Creative Commons Attribution 3.0 License, can be found at the PHP official website: http://php.net/manual/en/.
When a PHP file is requested from the server, the server first executes blocks of PHP code within the file. A ‘normal HTML file’ is then returned to the client’s browser. As such, there is no way for a casual visitor to read the PHP code. These blocks of code are contained by the symbols <?php … ?>:
The server, after replacing the PHP block with the output, returns a converted document to the browser. (echo and print output the value of an expression, and they can be used interchangeably.)
RESETRUNFULL
<!-- intro.php -->
<!DOCTYPE html>
<html>
<head></head>
<body><?php
   echo "Hello "; 
   print "<b>World!</b>";
?>
</body>
</html>

<!-- intro.php -->
<!DOCTYPE html>
<html>
<head></head>
<body>Hello <b>World!</b></body>
</html>
<!-- One can choose to return only certain parts of the document: -->

RESETRUNFULL
<!-- introd.php -->
<!DOCTYPE html>
<html>
<head></head>
<body>
   <?php
     $expression=true;
     if ($expression == true): ?>
    This part will be shown.
<?php else: ?>
    This part will not be shown.
<?php endif; ?> 
</body>
</html>

<!-- introd.php -->
<!DOCTYPE html>
<html>
<head></head>
<body>
 This part will be shown.
</body>
</html>
With PHP, it becomes possible to generate HTML code automatically. This is especially useful if the webpage contains a lot of repetitive HTML code segments.