Turing Machine

<?php
//Enter your code here, enjoy!

$input_string = '1 * 1 1';
$input_string_array = explode(' ', $input_string);


$cassette = array_merge(['a0'], $input_string_array, ['a0']);



$position = count($input_string_array) ;
$table = [
    'q1' => ['', ['q2', 'a0', 'l'], ['q0', 'a0', 's']],//['q3', '1', 'r'], ['q1', 'a0', 'l']
    'q2' => [['q3', '1', 'r'], ['q2', '1', 'l'], ['q2', '*', 'l']],
    'q3' => [['q1', 'a0', 'l'], ['q3', '1', 'r'], ['q3', '*', 'r']]
];

$q = 'q1';

$count_iter = 0;
//echo $cassette[$position] . ' ' . $q . PHP_EOL;

//print_r($table[$q][$cassette[$position]]);

while($q != 'q0') {
    if($count_iter > 500)
        break;
    $count_iter++;
    $current = $table[$q][toRow($cassette[$position])];
    $cassette[$position] = $current[1];
    $q = $current[0];
    if($current[2] == 'l') {
        $position--;
    }elseif($current[2] == 'r') {
        $position++;
    }
    if($position < 0) {
        $cassette = array_merge(['a0'], $cassette);
        $position = 0;
    }
    if($position > count($cassette))
        $cassette = array_merge($cassette, ['a0']);
    echo $count_iter . ') Cassette: ' . implode(' ', $cassette) . ' (' . $q . ')' . PHP_EOL;
}

function toRow($name) {
    $rows = ['a0' => 0, '1' => 1, '*' => 2];
    return $rows[$name];
}