Authenticate an external app through a Symfony2 + FOSUserBundle database

PasswordService.php

<?php

class PasswordService 
{
    public function __construct($algorithm = 'sha512', $encodeHashAsBase64 = true, $iterations = 5000) 
    {
        $this->algorithm = $algorithm;
        $this->encodeHashAsBase64 = $encodeHashAsBase64;
        $this->iterations = $iterations;
    }

    public function encodePassword($raw, $salt) 
    {
        $salted = $this->mergePasswordAndSalt($raw, $salt);

        $digest = hash($this->algorithm, $salted, true);

        for ($i = 1; $i < $this->iterations; $i++) {
            $digest = hash($this->algorithm, $digest.$salted, true);
        }

        return $this->encodeHashAsBase64 ? base64_encode($digest) : bin2hex($digest);
    }

    public function isPasswordValid($encoded, $raw, $salt) 
    {
        return $this->comparePasswords($encoded, $this->encodePassword($raw, $salt));
    }

    protected function mergePasswordAndSalt($password, $salt) 
    {
        if (empty($salt)) {
            return $password;
        }

        if (false !== strrpos($salt, '{') || false !== strrpos($salt, '}')) {
            throw new \InvalidArgumentException('Cannot use { or } in salt.');
        }

        return $password.'{'.$salt.'}';
    }

    protected function comparePasswords($password1, $password2) 
    {
        return self::equals($password1, $password2);
    }

    private static function equals($knownString, $userInput) 
    {
        $knownString .= chr(0);
        $userInput .= chr(0);

        $knownLen = strlen($knownString);
        $userLen = strlen($userInput);

        $result = $knownLen - $userLen;

        for ($i = 0; $i < $userLen; $i++) {
            $result |= (ord($knownString[$i % $knownLen]) ^ ord($userInput[$i]));
        }

        return 0 === $result;
    }
}