foreach ($dentists as $key => $dentists_index)
    {
        if($dentists_index["approved"] == 0)
        {
            unset($dentists[$key]);
        }
    }

<?
$dentists = array(
    array(
        'name' => 'kevin',
        'approved' => 0,
    ),
    array(
        'name' => 'cathy',
        'approved' => 0,
    ),
    array(
        'name' => 'steven',
        'approved' => 1,
    ),
);

foreach ($dentists as $key => $dentists_index) {
    print "Dentist #$key - Approved: " . $dentists_index['approved'] . "<br />n";

  if($dentists_index['approved'] == 0) {
    unset($dentists[$key]);
  }
}

var_dump($dentists);
?>