Generate a random string of non-ambiguous numbers and letters.

CodeGenerator.php

<?php

class CodeGenerator
{
    private static $alphabet   = '23456789abcdefghijkmnpqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ';
    private static $max_offset = 55;

    /**
     * Generate a random character of non-ambiguous number or letter.
     * @return string the character
     */
    public static function randomChar()
    {
        $index = mt_rand(0, self::$max_offset);
        return self::$alphabet[$index];
    }

    /**
     * Generate a random string of non-ambiguous numbers and letters.
     * @param $length Size of generated string
     * @return string
     * @throws InvalidArgumentException
     */
    public static function generate($length = 16)
    {
        if ($length < 1) {
            throw new InvalidArgumentException(__METHOD__ . ' expects a $length argument of at least 1, input was: ' . $length);
        }

        $random = '';

        for ($i=0; $i < $length; $i++) {
            $random .= self::randomChar();
        }

        return $random;
    }
}

CodeGeneratorTest.php

<?php

class CodeGeneratorTest extends PHPUnit_Framework_TestCase
{
    public function testRandomCode()
    {
        foreach (array(1, 10, 100) as $length) {
            $random = CodeGenerator::generate($length);
            $this->assertEquals($length, strlen($random));
        }
    }

    public function testZeroException()
    {
        $this->setExpectedException('InvalidArgumentException');
        CodeGenerator::generate(0);
    }

    public function testNegativeException()
    {
        $this->setExpectedException('InvalidArgumentException');
        CodeGenerator::generate(-1);
    }
}