I'm not sure where I found this, but I have been using it on a few projects recently and it's helped a lot. It basically detects what system you are on and will give you a constant that keeps hold of the slash for that system.

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if (strtoupper(substr(PHP_OS,0,3)) == 'WIN') {
 // Windows
 define('SLASH', '\\');
} else {
 // Linux/Unix 
 define('SLASH', '/');
}

For example, on a Windows system a file might be in C:\folders\data\, whereas on Linux the file would be in /folders/data/. So if you are given the full path as a string it can be difficult to separate the filename from the directory without knowing what system you are on.