Although it is possible to write and run standalone PHP programs, most PHP code is embedded in HTML or XML files. This is, after all, why it was created in the first place. Processing such documents involves replacing each chunk of PHP source code with the output it produces when executed.
Because a single file usually contains PHP and non-PHP source code, we need a way to identify the regions of PHP code to be executed. PHP provides four different ways to do this.
As you’ll see, the first, and preferred, method looks like XML. The
second method looks like SGML. The third method is based on ASP tags. The
fourth method uses the standard HTML
<script> tag; this makes it easy to edit
pages with enabled PHP using a regular HTML editor.
Because of the advent of the eXtensible Markup Language (XML) and the migration of HTML to an XML language (XHTML), the currently preferred technique for embedding PHP uses XML-compliant tags to denote PHP instructions.
Coming up with tags to demark PHP commands in XML was easy,
because XML allows the definition of new tags. To use this style,
surround your PHP code with
?>. Everything between these
markers is interpreted as PHP, and everything outside the markers is
not. Although it is not necessary to include spaces between the markers
and the enclosed text, doing so improves readability. For example, to
get PHP to print “Hello, world,” you can insert the following line in a
<?php echo "Hello, ...