Comments and Comment Blocks in PHP

PHP supports C language, C++ programming language and Unix shell-style (Perl style) comments. In PHP, // or # is used to make a single-line comment or /* and */ to make a large comment block that spans several lines.

For example:

<?php 

echo 'This is a test'; //This is a single-line C++ style comment
echo 'This is another test'; #This is another single-line shell-style comment
/* This is a multi line comment
Second line of comment */

?>

The “one-line” comment styles comment to the end of the line or the current block of PHP code, whichever comes first. This means that HTML code after the PHP script block (PHP mode broken with ?> and returns to HTML mode after end of PHP code block) will be printed. If the asp_tags configuration directive is enabled, it behaves the same with // %> and # %>. However, the </script> tag doesn’t break out of PHP mode in a single-line comment.

For example,

<h1>This is an <?php # echo 'simple';?> example.<h1>

<p>The header above will say 'This is an  example'.</p>

C or C++ style comments end at the first */ encountered. Any nested /* and */ style comments will cause error, and it’s pretty easy to make this mistake if you are trying to comment out a large block of code.

<?php

/* 
echo 'This is a test'; /* This comment will cause a problem */
*/

?>

The PHP comments is useful when you want to comment or disable a large portion of HTML. Instead comment the HTML code line by line, put the whole HTML block into PHP code block (i.e, add <?php and ?>) and comment with /* and */.