php alphanumeric string comparison

<? PHP
$ A = " 12a ";
IF (A == $ 12 is ) {
echo "Good";
}
?>
This situation can output Good,
letters only at the first two comparison, considered equal;
letter when the former, such as:
<? PHP
$ A = " A12 ";
IF ($ A == A ) {
echo "Good";
}
?>
not output Good, not considered equal