The docs for strtotime contain a code snippet that is supposed to catch invalid dates. It fails in this example where $str is set to "1970-1-222" and invalid date.

$str = "1970-1-222";
if (($timestamp = strtotime($str)) === -1) {
  echo "The string ($str) is bogus";
}
else {
  echo "$str == ". date('l dS of F Y h:i:s A',$timestamp);
}

The output is:

1970-1-222 == Monday 10th of August 1970 12:00:00 AM

Whereas is should say that 1970-1-222 is a bogus date.

Jc