Casting Weirdness - Possible PHP BUG?

I don't know why I'm getting weird results here. Maybe someone can shed some light? (Please see code below) On my box I am getting the following output: Invalid Valid However, it SHOULD be: Valid Valid ... shouldn't it? If I place the 0 in $valid_chars1 anywhere but the beginning or end: $valid_chars1 = 1234567809; Then it works, but inspection of $valid_chars1 after it has been cast to (string) shows "0123456789". What gives? Why isn't strstr() seeing it? Is this a PHP bug? CODE BELOW ---------------------------------------------------------------------------- ------------ function validate($value, $valid_chars) { for ($i = 0; $i <= strlen($value) - 1; $i++) { if (!strstr($valid_chars, substr($value, $i, 1))) return false; } return true; } $value1 = 123405; $value1 = (string) $value1; $valid_chars1 = 0123456789; $valid_chars1 = (string) $valid_chars1; $value2 = '123405'; $valid_chars2 = '0123456789'; print validate($value1, $valid_chars1) ? 'Valid' : 'Invalid'; print "\n"; print validate($value2, $valid_chars2) ? 'Valid' : 'Invalid';