Ranking words of file texts

script.php

<?php

// Read file contents.
$path = 'texts-youtube/';
$content = '';

if ($handle = opendir($path)) {
    while (false !== ($entry = readdir($handle))) 
        if ($entry != "." && $entry != "..") 
            $content .= file_get_contents($path . $entry);

    closedir($handle);
}

// Prepare words.
function check_letter ($ch) {
    $letters = array(
        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 
        'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'
    );

    for ($a=0; $a<count($letters); $a++)
        if ($letters[$a] == $ch) 
            return true;

    return false;
}

$words = explode(" ", $content);
$newWords = array();

for ($a=0; $a<count($words); $a++) {
    $word = trim($words[$a]);
    $word = strtolower($word);
    $tmp  = '';

    for ($b=0; $b<strlen($word); $b++) {
        if (check_letter($word[$b]) === true)
            $tmp .= $word[$b];
    }

    $word = trim($tmp);

    if (!empty($word) && strlen($word) >= 2)
        $newWords []= $word;
}

// Remove duplicate.
$words = $newWords;
$wordlist = array();
$total = count($words);

echo "Remove duplicate...\n";
echo "Current total: ". $total ."\n";

for ($a=0; $a<count($words); $a++) {
    $found = false;
    $index = -1;

    for ($b=0; $b<count($wordlist); $b++) {
        if ($words[$a] == $wordlist[$b]['word']) {
            $found = true;
            $index = $b;
            break;
        }
    }

    if ($found === false)
        $wordlist []= array(
            'word' => $words[$a],
            'total' => 1
        );

    else
        $wordlist[ $index ]['total']++;

    echo "\rProcess: ". $a ." / ". $total;
}

echo "\n";

// Order list.
$words = $wordlist;
$total = array_column($words, 'total');
array_multisort($total, SORT_DESC, $words);

for ($a=0; $a<count($words); $a++)
    file_put_contents('words.txt', $words[$a]['total'] .' - '. $words[$a]['word'] ."\n", FILE_APPEND);

echo "Finished!\n\n\n";