These are functions I knew about for some time but never really used, it's very useful though.

Function: htmlspecialchars_decode()

What you might have been using: Something along the lines of str_replace('>','>',$string);

Why you should use this: less lines of code

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Constant: PHP_EOL

What you might have been using: "\r\n" or "\n", depending on your platform

Why you should use this: portability! Your file reading code suddenly becomes compatible with both Windows and Linux based servers.

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Function: nl2br()

What you might have been using: str_replace("\n","<br />",$string) or str_replace(PHP_EOL,"<br />",$string) at best

Why you should use this: nl2br does all the work for you, it means "newline to <br />", what more do you need?

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Function: wordwrap()

What you might have been using: too long to write down

Why you should use this: does all the work for you again, it is perfect for wrapping long texts to a readable format instead of forcing your users to scroll.