The simplest approach would be:
preg_match('/[א-ת]/',$string)
For example,
$strings = array( "abbb","1234","aabbאאבב","אבבבב");
foreach($strings as $string)
{
echo "'$string' ";
echo (preg_match('/[א-ת]/',$string))? "has Hebrew characters in it." : "is not Hebrew";
echo "
";
}