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注释

PHP 支持 C，C++ 和 Unix Shell 风格（Perl 风格）的注释。例如:

<?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
?>

单行注释仅仅注释到行末或者当前的 PHP 代码块，视乎哪个首先出现。这意味着在 // ... ?> 或者 # ... ?> 之后的 HTML 代码将被显示出来：?> 跳出了 PHP 模式并返回了 HTML 模式，// 或 # 并不能影响到这一点。如果启用了 asp_tags 配置选项，其行为和 // %> 或 # %> 相同。不过，</script> 标记在单行注释中不会跳出 PHP 模式。

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

C 风格的注释在碰到第一个 */ 时结束。要确保不要嵌套 C 风格的注释。试图注释掉一大块代码时很容易出现该错误。

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

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用户评论:

team at researchbib dot com (14-Sep-2011 03:25)

when the comment string contains '?>', you should be careful.

e.g. output code 1= code 2 is different with code 3
1. with //
<?php

// echo '<?php ?>';

?>

2. with #

<?php

// echo '<?php ?>';

?>

3. with /* */

<?php

/* echo '<?php ?>';*/

?>

philip-php at dago dot yourweb dot de (05-Mar-2011 06:29)

It's true, comments do not take up PROCESSING time, but they do take some PARSING time in case you are not using a compile cache of some kind.

jballard at natoga dot com (15-Dec-2010 10:28)

Comments do NOT take up processing power.

So, for all the people who argue that comments are undesired because they take up processing power now have no reason to comment ;)

<?php

// Control
echo microtime(), "<br />"; // 0.25163600 1292450508
echo microtime(), "<br />"; // 0.25186000 1292450508

// Test
echo microtime(), "<br />"; // 0.25189700 1292450508
# TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST TEST
# .. Above comment repeated 18809 times ..
echo microtime(), "<br />"; // 0.25192100 1292450508

?>

They take up about the same amount of time (about meaning on a repeated testing, sometimes the difference between the control and the test was negative and sometimes positive).

benny at bennyborn dot de (08-Dec-2010 11:16)

This regex should do the job when trying to parse comments

(\/\*(.*?)\*\/)|(^|\s+)\/\/(.*?)(\n|$)|(^|\s+)#(.*?)(\n|$)

Wolfsbay at ya dot ru (12-May-2010 10:10)

If you are using editor with code highlight, it’s much easier to notice error like /* */ */.

theblazingangel at aol dot com (28-Aug-2007 11:55)

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.

fun at nybbles dot com (14-Jul-2006 06:28)

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");
         }
?>

mst_NO_SPAM_TO_ME at mstsoft dot com (05-Jun-2006 01:38)

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 (26-May-2006 07:39)

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).

samuli dot karevaara at lamk dot fi (11-Nov-2005 04:30)

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 at wanadoo dot nl (29-Jun-2005 09:51)

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<br>
* in your development cycle save you a lot of time by preventing you having to rewrite<br>
* major documentation parts to generate some usable form of documentation.
*/
?>
Some basic html-like formatting is supported with this (ie <br> tags) to create something of a layout.

M Spreij (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:
<?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;
}
// */
?>

Steve (15-Dec-2004 12:41)

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.*/');

*/

?>