Узнать частоту повторения различных слов в тексте

<?php
set_time_limit(0);
$arr_file = file("key.txt");
$str_file = str_replace("\r\n"," ",implode("\r\n",$arr_file));
$arr_key = explode(" ",$str_file);
foreach($arr_key as $value)
{
if(strlen($value)>4)
{
preg_match_all("#$value#","$str_file",$key);
if(count($key[0])>=2)
{
$arr_viborki[] = $value;
}
}
}
$arr_viborki = array_unique($arr_viborki);
foreach($arr_viborki as $val)
{
foreach($arr_file as $val2)
{
if(preg_match("#$val#","$val2"))
{
file_put_contents("выборки/$val.txt",$val2,FILE_APPEND);
}
}
}
echo "<h3>Готово</h3>";
?>