PHP as well as most (all?) other programming languages provide different ways to comment source code.
Most common comment indicators are # , // and /* */ .

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# this is a comment
echo 'print something';
// another comment
phpinfo();
/* and one more */
var_dump($_ENV);

Which one should I use?

# vs //

I definitely tend to # since it’s only one character instead of two slashes.

# and // vs. /* */

If you plan to pack your code I would highly recommend to always use /* */ due to it’s source code packer compatibility.

/* */ also provides multi line commenting.

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/**
* this
* is a
* valid comment
**/

So long,
JaZz