Difference between mb_substr and substr

string substr ( string $string , int $start [, int $length ] )

Returns the portion of string specified by the start and length parameters.

string mb_substr ( string $str , int $start [, int $length [, string $encoding ]] )

Performs a multi-byte safe substr() operation based on number of characters. Position is counted from the beginning of str. First character's position is 0. Second character position is 1, and so on.

It will impact your script if you work with multi-byte text that you substring from. If this is the case, I higly recommend enabling mb_* functions in your php.ini or do this ini_set("mbstring.func_overload", 2);

If you have utf-8 encoding use mb_substr

Example :

echo substr("hi mémé", 0 , 5); // will print hi m�
echo mb_substr("hi mémé", 0 , 5); // will print hi mé