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Notes can come in all sorts of shapes and sizes. They vary, and their uses are completely up to the person writing the code. However, I try to keep things consistent in my code that way it's easy for the next person to read. So something like this might help...

<?php

//======================================================================
// CATEGORY LARGE FONT
//======================================================================

//-----------------------------------------------------
// Sub-Category Smaller Font
//-----------------------------------------------------

/* Title Here Notice the First Letters are Capitalized */

# Option 1
# Option 2
# Option 3

/*
 * This is a detailed explanation
 * of something that should require
 * several paragraphs of information.
 */
 
// This is a single line quote.
?>

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;
}
// */
?>

As of php 8, single line comments starting exactly with "#[" have a special meaning: they are treated as "attributes", and they must respect the expected syntax. See: https://www.php.net/manual/en/language.attributes.php

So the following code throws an error in php 8+, while it is perfectly valid in php <8:
<?php
#[~~my super cool comment~~~]
?>

To be safe, just always use "//" comments instead of "#". Maybe in the future there will be other special meanings for the "#" comments, who knows.

It is worth mentioning that, HTML comments have no meaning in PHP parser. So,

<!-- comment
<?php echo some_function(); ?>
-->

WILL execute some_function() and echo result inside HTML comment.

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.

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 */

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.*/');

*/

?>

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.

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).

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

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");
         }
?>