Most of the popular languages (C, C++, Unix) uses same kind of comments, single line (//) and more than one line comments (/* */) and along with these Unix like comment '#' is also supported by PHP.
Single line comments ( // ) is used to make a single line or a block inactive. Similarly the task of # is the same.
Double line or multiple line comments are used when large block is to be commented.
//This is a single comment
#This is Unix style comment
Note: Since the above example has only comments, no output will be generated
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