pangrams

<?php

/*
 * Complete the 'pangrams' function below.
 *
 * The function is expected to return a STRING.
 * The function accepts STRING s as parameter.
 */

function pangrams($check_sentence)
{
    // initial letter mapping
    $all_pangrams_letter_mapping = [];
    $ascii_from = 97;
    $ascii_to = 122;
    for ($ascii_code = $ascii_from; $ascii_code <= $ascii_to; $ascii_code++) {
        $letter = chr($ascii_code);
        $all_pangrams_letter_mapping[$letter] = true;
    }


    // check every letter
    $total_length_of_sentence = strlen($check_sentence);
    for ($letter_index = 0; $letter_index < $total_length_of_sentence; $letter_index++) {
        // get letter
        $letter = $check_sentence[$letter_index];
        // convert to lower case
        $letter = strtolower($letter);
        unset($all_pangrams_letter_mapping[$letter]);
    }

    // calculate left letter count
    $left_letter_count = count($all_pangrams_letter_mapping);

    $check_pangram = 'pangram';
    if ($left_letter_count > 0) {
        $check_pangram = 'not pangram';
    }

    return $check_pangram;
}

$fptr = fopen(getenv("OUTPUT_PATH"), "w");

$s = rtrim(fgets(STDIN), "\r\n");

$result = pangrams($s);

fwrite($fptr, $result . "\n");

fclose($fptr);