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PHP : Language Reference : Basic syntax : Comments

Comments

PHP supports 'C', 'C++' and Unix shell-style (Perl style) comments. For example:

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<?php
   echo 'This is a test'; // This is a one-line c++ style comment
   /* This is a multi line comment
      yet another line of comment */
   echo 'This is yet another test';
   echo 'One Final Test'; # This is a one-line shell-style comment
?>

The "one-line" comment styles only comment to the end of the line or the current block of PHP code, whichever comes first. This means that HTML code after // ... ?> or # ... ?> WILL be printed: ?> breaks out of PHP mode and returns to HTML mode, and // or # cannot influence that. 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 one-line comment.

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<h1>This is an <?php # echo 'simple';?> example.</h1>
<p>The header above will say 'This is an  example'.</p>

'C' style comments end at the first */ encountered. Make sure you don't nest 'C' style comments. It is easy to make this mistake if you are trying to comment out a large block of code.

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<?php
/*
   echo 'This is a test'; /* This comment will cause a problem */
*/
?>

Related Examples ( Source code ) » language.basic_syntax.comments

Examples ( Source code ) » Shell style comments
Examples ( Source code ) » PHP comments are usually preceded by double slashes

Code Examples / Notes » language.basic_syntax.comments

mst_no_spam_to_me
This "comment ends on line break or end of PHP Block" thing can be confusing. I discovered this by accident when working with XML Output from PHP...
<?PHP
header("Content-type: text/xml");
/*
echo "<?xml version=\"1.0\"?>";
echo "<page>multi-line comments work as expected.</page>";
*/
//echo "<?xml version=\"1.0\"?>";
//echo "<page>single-line comments end php mode and output your code.</page>";
?>
I would expect the comment to work, but there is no parsing in comments so the String suddenly becomes a PHP  end-block tag, which is correct reading this documentation.
cheers,
martin
PS: You even see the behavior in the Syntax highlighting :-)

j lee
MSpreij (8-May-2005) says  /* .. */ overrides //  
Anonymous (26-Jan-2006) says // overrides /* .. */
Actually, both are correct. Once a comment is opened, *everything* is ignored until the end of the comment (or the end of the php block) is reached.
Thus, if a comment is opened with:
  //  then /* and */ are "overridden" until after end-of-line
  /*  then // is "overridden" until after */

21-jan-2006 09:46
M Spreij wrote, 08-May-2005 08:15...
A nice way to toggle the commenting of blocks of code can be done by mixing the two comment styles:
...
This works because a /* .. */ overrides //.
The final sentence should be the other way round, i.e.
This works because a // overrides /* .. */.
(If it didn't the /* .. */ would comment out the code regardless of whether an additional '/' is prefixed to the first line).

theblazingangel
it's perhaps not obvious to some, but the following code will cause a parse error! the ?> in //?> is not treated as commented text, this is a result of having to handle code on one line such as <?php echo 'something'; //comment ?>
<?php
if(1==1)
{
//?>
}
?>
i discovered this "anomally" when i commented out a line of code containing a regex which itself contained ?>, with the // style comment.
e.g. //preg_match('/^(?>c|b)at$/', 'cat', $matches);
will cause an error while commented! using /**/ style comments provides a solution. i don't know about # style comments, i don't ever personally use them.

samuli dot karevaara
If you want to comment out large sections of code (temporarily, usually and hopefully), consider using
<?php
if (0) {
    print("This code is 'commented' out");
}
?>
instead of /* comment block */. Otherwise, as noted here, you will have parse errors if the block that you commented out contains */ somewhere, like in regexp or in another comment.

hcderaad
Comments in PHP can be used for several purposes, a very interesting one being that you can generate API documentation directly from them by using PHPDocumentor (http://www.phpdoc.org/).
Therefor one has to use a JavaDoc-like comment syntax (conforms to the DocBook DTD), example:
<?php
/**
* The second * here opens the DocBook commentblock, which could later on
* in your development cycle save you a lot of time by preventing you having to rewrite
* major documentation parts to generate some usable form of documentation.
*/
?>
Some basic html-like formatting is supported with this (ie
tags) to create something of a layout.

steve
Be careful when commenting out regular expressions.
E.g. the following causes a parser error.
I do prefer using # as regexp delimiter anyway so it won't hurt me ;-)
<?php
/*
$f->setPattern('/^\d.*/');
*/
?>

fun
a trick I have used in all languages to temporarily block out large sections (usually for test/debug/new-feature purposes), is to set (or define) a var at the top, and use that to conditionally comment the blocks; an added benefit over if(0) (samuli's comment from nov'05) is that u can have several versions or tests running at once, and u dont require cleanup later if u want to keep the blocks in:  just reset the var.
personally, I use this more to conditionally include code for new feature testing, than to block it out,,,, but hey, to each their own :)
this is also the only safe way I know of to easily nest comments in any language, and great for multi-file use, if the conditional variables are placed in an include :)
for example, placed at top of file:
<?php $ver3 = TRUE;  
      $debug2 = FALSE;
?>
and then deeper inside the file:
<?php if ($ver3) {
          print("This code is included since we are testing version 3");
        }
?>
<?php if ($debug2) {
          print("This code is 'commented' out");
        }
?>

m spreij
A nice way to toggle the commenting of blocks of code can be done by mixing the two comment styles:
<?php
//*
if ($foo) {
 echo $bar;
}
// */
sort($morecode);
?>
Now by taking out one / on the first line..
<?php
/*
if ($foo) {
 echo $bar;
}
// */
sort($morecode);
?>
..the block is suddenly commented out.
This works because a /* .. */ overrides //. You can even "flip" two blocks, like this:
<?php
//*
if ($foo) {
 echo $bar;
}
/*/
if ($bar) {
 echo $foo;
}
// */
?>
vs
<?php
/*
if ($foo) {
 echo $bar;
}
/*/
if ($bar) {
 echo $foo;
}
// */
?>

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