For a german t9 keypad. currently no checking for breaks between pressed characters.

t9-decoder.php

<?php

declare(strict_types=1);

/* german T9 */
$str = '9335552224433 "2" 8444833555 4222 337777 9999886 5552886622244 3337777 6644466833663666 "64" 44466 3338887777222445552663?';

$chars = range('a', 'z');
$fourCharKeys = [7,9];

$decodingTable = [];
$charPosition = 0;
foreach (range(2, 9) as $number) {
    $numberCharCount = in_array($number, $fourCharKeys) ? 4 : 3;
    for ($i=1; $i <= $numberCharCount; $i++) {
        $decodingTable[$chars[$charPosition++]] = str_repeat((string)$number, $i);
    }
}
// replace "multiple-pressed button" letters first
arsort($decodingTable);

echo str_replace(array_values($decodingTable), array_keys($decodingTable), $str);