Count words from text file

wordcounter.php

<?php
/*
 * Count number of words from the given txt file
 * @author Antonis Flangofas
 * @date 18/08/2014
 */ 
class WordCounter 
{
    const ASC = 2;
    const DESC = 4;

    private $word_count;

    function __construct($file, $flag = '') 
    {
        if (!is_readable($file)) {
            throw InvalidArgumentException('File does not exist!');
        }       
        //get text and add all words to an array then count the values
        $this->word_count = array_count_values(str_word_count(strtolower(file_get_contents($file)), 1));

        return $this->word_count;
    }

    /*
     * Sort array if needed either ASC or DESC using bitwise operators
     */ 
    final public function sortByOccurrence($flag) {
        if ($flag & static::ASC) {
            asort($this->word_count);
        }
        if ($flag & static::DESC) {
            arsort($this->word_count);
        }

        return $this->word_count;
    }
}