Comment in PHP are usually written within the block of PHP code to 
describe the functionality or let others understand of your code. 
Comments are not execute, means not show in the output. 
There are Two types of comments.

Single Line Comment
Multi Line Comment

1) Single Line:
<?php
    echo "Hello World!"; // Output "Hello World!"
?>

2) Multi Line:
<?php
    /* The following line of code
       will output the "Hello World!" message */
    echo "Hello World!";
?>