$identSize = preg_match_all("/^( )[^ ]/",$line,$matches);

$example1 = " Foo"; // should return 1
$example2 = "  Bar"; // should return 2
$example3 = "   Foo bar"; // should return 3,not 4!

$identSize = strlen($line)-strlen(ltrim($line));

preg_match('/^(\s+)/',$matches);
$identSize = strlen($matches[1]);