When PHP parses a file, it looks for opening and closing tags, which tell PHP to start and stop interpreting the code between them. Parsing in this manner allows php to be embedded in all sorts of different documents, as everything outside of a pair of opening and closing tags is ignored by the PHP parser. Most of the time you will see php embedded in HTML documents, as in this example.
You can also use more advanced structures:
There are four different pairs of opening and closing tags which can be used in php. Two of those, <?php ?> and <script language="php"> </script>, are always available. The other two are short tags and ASP style tags, and can be turned on and off from the php.ini configuration file. As such, while some people find short tags and ASP style tags convenient, they are less portable, and generally not recommended.
Note: Also note that if you are embedding PHP within XML or XHTML you will need to use the <?php ?> tags to remain compliant with standards.
Example 10-2. PHP Opening and Closing Tags
While the tags seen in examples one and two are both always available, example one is the most commonly used, and recommended, of the two.
Short tags (example three) are only available when they are
enabled via the short_open_tag
php.ini configuration file directive, or if php was configured
Note: If you are using PHP 3 you may also enable short tags via the short_tags() function. This is only available in PHP 3!
ASP style tags (example four) are only available when they are enabled via the asp_tags php.ini configuration file directive.
Note: Support for ASP tags was added in 3.0.4.
Note: Using short tags should be avoided when developing applications or libraries that are meant for redistribution, or deployment on PHP servers which are not under your control, because short tags may not be supported on the target server. For portable, redistributable code, be sure not to use short tags.