If your requirements are as simple as you seem to state, displaying positive integer keys as numbers, here's a simple solution: 
<?php
global $level_0, $level_1, $level_2, $level_3;
$level_0=array('','one','two','three','four','five','six','seven',
  'eight','nine','ten','eleven','twelve','thirteen','fourteen',
  'fifteen','sixteen','seventeen','eighteen','nineteen');
$level_1=array(2=>'twenty',3=>'thirty',4=>'forty',5=>'fifty',
  6=>'sixty',7=>'seventy',8=>'eighty',9=>'ninety');
$level_2='hundred';
$level_3='thousand';
function gn($num){
  global $level_0, $level_1, $level_2, $level_3;
  $end='';
  $arrnum=array_reverse(str_split(strval($num)));
  $start='';
  if($num%100>0)$end=f99($num);
  for($i=strlen(strval($num))-1; $i>1; $i--){
    if($i==4) $start.=f99($arrnum[$i--].$arrnum[$i]).${'level_'.$i};
    else {
      $break=false;
      if($num%pow(10,$i)==0) $break=true;
      if($level_0[$arrnum[$i]]){
        $start.=$level_0[$arrnum[$i]].${'level_'.$i};
      }
      if($break) break;
    }
  }
  return $start.$end;
}
function f99($num){
  $arrnum=array_reverse(str_split(strval($num)));
  global $level_0, $level_1;
  $num=$num%100; //ensure 2 digits integer
  $end='';
  if($num%100<20)$end=$level_0[$num%100];
  else {
    $end=$level_1[$arrnum[1]];
    if($num%10>0) $end.=$level_0[$arrnum[0]];
  }
  return $end;
}
// Test statements
echo "12 - ".gn(12)." \n";
echo "20 - ".gn(20)." \n";
echo "37 - ".gn(37)." \n";
echo "50 - ".gn(50)." \n";
echo "99 - ".gn(99)." \n";
echo "125 - ".gn(125)." \n";
echo "160 - ".gn(160)." \n";
echo "300 - ".gn(300)." \n";
echo "2365 - ".gn(2365)." \n";
echo "1000 - ".gn(1000)." \n";
echo "1300 - ".gn(1300)." \n";
echo "10000 - ".gn(10000)." \n";
echo "30000 - ".gn(30000)." \n";
echo "23653 - ".gn(23653)." \n";
If you need higher numbers, you can include new levels in the gn-function.