When PHP parses a file, it simply passes the text of the file through until it encounters one of the special tags which tell it to start interpreting the text as PHP code. The parser then executes all the code it finds, up until it runs into a PHP closing tag, which tells the parser to just start passing the text through again. This is the mechanism which allows you to embed PHP code inside HTML: everything outside the PHP tags is left utterly alone, while everything inside is parsed as code.
There are four sets of tags which can be used to denote blocks of PHP code. Of these, only two (<?php. . .?> and <script language="php">. . .</script>) are always available; the others can be turned on or off from the php.ini configuration file. While the short-form tags and ASP-style tags may be convenient, they are not as portable as the longer versions. Also, if you intend to embed PHP code in XML or XHTML, you will need to use the <?php. . .?> form to conform to the XML.
The tags supported by PHP are:
Example 5-1. Ways of escaping from HTML
The first way is only available if short tags have been enabled. This can be done via the short_tags() function (PHP 3 only), by enabling the short_open_tag configuration setting in the PHP config file, or by compiling PHP with the --enable-short-tags option to configure.
Again, the second way is the generally preferred method, as it allows for the the use of PHP in XML-conformant code such as XHTML.
The fourth way is only available if ASP-style tags have been enabled using the asp_tags configuration setting.
Note: Support for ASP-style tags was added in 3.0.4.
The closing tag for the block will include the immediately trailing newline if one is present. Also, the closing tag automatically implies a semicolon; you do not need to have a semicolon terminating the last line of a PHP block.
PHP allows you to use structures like this: