World Wide Tutorial

What are comments?
Syntax of comments
Commenting out
1. What are comments?

Text or lines in your php scripts which are ignored by the php interpreter are called comments. This is like the html comments with <! - ->. With these you can (as the name implies it) write comments inside your script. These are used to add eg the authors name or the license into the php script without interfering the php interpreter.
2. Syntax of comments

There are 4 types of comments in php but we only use 3 of them. And from them two are almost the same. These comment types are separated into two groups, the one line comments and the multi-line comments

One line comments are comments which ends at the current line or at the>. These comments start with / / (this is not the string escape sequence \ \). The following text is marked as a comment and is ignored by the php interpreter.
<? php
echo 'text', / / echo a string

/ / Generate a second text
echo 'Another text';

/ / Echo 'I'm not printed as I am inside a comment';

echo 'I am printed, Although I got / / in the string';

/ / Comment running until the end php?> <? Php echo 'I' m not in the comment anymore;?>

As you see not every charactersequence of / / is treated as a comment, eg if it is used inside a string

Multi-line comments are comments which can be longer than one line. Instead of starting each line with a / / a multi line comment starts with a single / * and stops with a * /.
<? php
echo "print something again";
/ * Some comments which
use several
lines
until the ending * /
echo 'A normal php statement again';
?>

For readability each comment can be written as you want, like indenting the lines.
<? php
echo "print something again";
/ * Some comments which
Use several
Lines
Until the ending * /
echo 'A normal php statement again';
?>

You can even exaggerate this to underline the comments.
<? php
/*************************
* Loading configuration *
*************************/
echo "config loaded";
?>

For multi-line comments you can not use> to stop such a comment.

The third type are comments for PHPDoc. These are almost identically to multiline comments but uses / ** as the start of the comment. However it must be appended by a whitespace (typically a newline).
<? php
/ **
A comment in PHPDoc style
* /
echo "php normal code";
?>

You may ask why there are two almost identically comment tyes. PHPDoc are special comments which are used to document special parts of your php script. These are used before selfwritten functions.
<? php
/ **
* Output a number.
*
* The following function outpus the given number.
*
* @ Param i The number to print.
* /
function output_number ($ i) (
/ / ...
)
?>

The syntax of such PHPdocs is defined by the phpDocumentor project. It's much the same as the Javadoc tool with the same idea. Special Program (mentioned this PHPDoc) read this comments and generate html files for documentation. This way other php programmers can easily read the documentation to understand what the mentioned function (or class) is doing.
3. Commenting out

Besides writing developer notes into comments then you can do stuff which is called commenting out. During the development sometimes you want to disable some part of the script but without deleting the code. You can do this by putting the php code inside a comment. Check the following example.
<? php
do_this ();
do_that ();
and_this ();
?>

If you dont want to call do_that () just put it inside a one-line comment.
<? php
do_this ();
/ / do_that ();
and_this ();
?>

For more than one lines you still can use multi-line comments.
<? php
do_this ();
/ *
do_this ();
and_this ();
and_that ();
and_now_this ();
* /
and_again_that ();
?>

There is a trick to the active commands in the comment again almost, instead of just deleting the / * and * /. For this you must put the * / inside a one-line comment with / /.
<? php
do_this ();
/ *
do_this ();
and_this ();
and_that ();
and_now_this ();
/ / * /
and_again_that ();
?>

To reactive the execution of the statements replace the / * with a / / *.
<? php
do_this ();
/ / *
do_this ();
and_this ();
and_that ();
and_now_this ();
/ / * /
and_again_that ();
?>

Now you see the reason for the / / before the * /, otherwise you got invalid php code (and wants to get parse error). Now you have 2 one single line comments. If you replace / / * back to / * the statements are commented out again and you have one multi-line comment.

As a multiline comment ends with * / its impossible to embed multi-line comments inside multi-line comments. This is not a problem as you normaly dont do this explicit. But it can happend if you comment out some code and some days later comment out a wide area.
<? php
do_this ();
do_that ();
/ *
deactivated ();
deactivated2 ();
* /
and_this ();
and_that ();
?>

Now we want to deactivate the code from do_that () to and_this (). You can try this with a multiline comment.
<? php
do_this ();
/ *
do_that ();
/ *
deactivated ();
deactivated2 ();
* /
and_this ();
* /
and_that ();
?>

With this php code will stop the execution of the script. As you see in the syntax highlighting in this tutorial the outer comment is not really in outer comment. For the php multi line comments start at line 3 and ends at line 8 (and not at line 10). In this case even the line and_this () gets executed. However, the whole script is not run at all as the script results in a parse error in line 10th
Questions about the chapter
Which comment types exists?
1. Which comment types exists?

There are 4 types, but we handled only 3 of them. These are one line comments, multiline comments and PHPDoc comments. The 4th type are one line comments which uses # as the start instead of / /.