I was asked in a job interview to count all words in a string ignoring capitals and special characters

countWords.php

<?php
$str = "About nine o'clock the moon was sufficiently bright for me to proceed on my way and I had no difficulty in following the trail at a fast walk, and in some places at a brisk trot until, about midnight, I reached the water hole where Powell had expected to camp. I came upon the spot unexpectedly, finding it entirely deserted, with no signs of having been recently occupied as a camp.
I was interested to note that the tracks of the pursuing horsemen, for such I was now convinced they must be, continued after Powell with only a brief stop at the hole for water; and always at the same rate of speed as his.
I was positive now that the trailers were Apaches and that they wished to capture Powell alive for the fiendish pleasure of the torture, so I urged my horse onward at a most dan1gerous pace, hoping against hope that I would catch up with the red rascals before they attacked him.
Further speculation was suddenly cut short by the faint report of two shots far ahead of me. I knew that Powell would need me now if ever, and I instantly urged my horse to his topmost speed up the narrow and difficult moun!tain trail.";


/**
 * YOUR CODE GOES HERE
 */
function countWords($str) {

    // Ensure all lowercase
    $str = strtolower($str);

    // Strip carriage return from string and replace with space
    $str = str_replace("\n", ' ', $str);

    // Remove all non alphanumeric chars
    $str = preg_replace("/[^A-Za-z0-9 ]/", '', $str);

    // Create array
    $arr = explode(' ', $str);

    // Count values
    $arr = array_count_values($arr);

    // Sort high - low
    arsort($arr);

    return $arr;
}
print_r(countWords($str));