separate string into halves?Can i?

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Will the string always be in the same format? - If so you can use a regular expression:

$string = '1234123456abcd';

$search = "/^([0-9]{4})([0-9]{6})([a-z]{4})$/i";
$replace = '\1-\2-\3';

$string = preg_replace($search, $replace, $string);