Description:
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A quite often used code is to check if a string / binary string begins with a certain substring. Using substr() requires setting 2nd param always to 0 and 3rd param to length, which is the often set hardcoded or with strlen() / mb_strlen(). Example:

substr($str, 0, 7) == "http://"
// or
substr($str, 0, strlen("http://")) == "http://"

This can lead to errors, when user sets wrong length, mixes substr() & strlen() with their multibyte versions, etc. A shorter version could be done with strpos() and checking for position 0, but strpos() searches the entire string. RegEx has performance implications.

A simple function would result in a cleaner and a more robust and readable code. Example:

str_begins($str, $substr)  // => true / false

Its performance can also be optimized quite good, I think (compared to substr()).