Simple function mocker for PHP 5.3

FunctionMocker.php

<?php
class FunctionMocker
{
    /** @var FunctionMocker[] */
    private static $instances = array();

    /** @var string */
    private $namespace;

    /** @var string */
    private $function;

    /** @var callable $callback */
    private $callback;

    /** @var int */
    private $numberOfCalls = 0;

    /**
     * @param string $namespace
     * @param string $function
     */
    private function __construct($namespace, $function)
    {
        $this->namespace = (string) $namespace;
        $this->function = (string) $function;
    }

    /**
     * @param string $namespace
     * @param string $function
     * @return FunctionMocker
     */
    public static function getInstance($namespace, $function)
    {
        $name = sprintf('%s\%s', $namespace, $function);

        if (!isset(self::$instances[$name])) {
            self::$instances[$name] = new self($namespace, $function);
        }

        return self::$instances[$name];
    }

    /**
     * @return int
     */
    public function getNumberOfCalls()
    {
        return $this->numberOfCalls;
    }

    public function resetNumberOfCalls()
    {
        $this->numberOfCalls = 0;
    }

    /**
     * @param callable|null $callback
     * @throws \InvalidArgumentException
     */
    public function mock($callback = null)
    {
        $this->callback = $callback ?: $this->function;
        $function = sprintf('\%s\%s', $this->namespace, $this->function);

        if (!is_callable($this->callback, false, $name)) {
            throw new \InvalidArgumentException(sprintf('"%s" is not callable', $name));
        }

        if (function_exists($function)) {
            throw new \RuntimeException(sprintf(
                'The function "%s" already exists. It can be mocked only once per namespace.',
                $function
            ));
        }

        eval(sprintf(
            '
                namespace %s;

                function %s()
                {
                    $functionMocker = \%s::getInstance(__NAMESPACE__, "%s");

                    return $functionMocker->call(func_get_args());
                }
            ',
            $this->namespace,
            $this->function,
            __CLASS__,
            $this->function
        ));
    }

    /**
     * @param array $arguments
     * @return mixed
     * {@internal}
     */
    public function call(array $arguments)
    {
        $this->numberOfCalls++;

        return call_user_func_array($this->callback, $arguments);
    }
}

test.php

<?php
namespace Test;

$timeMocker = \FunctionMocker::getInstance(__NAMESPACE__, 'time');
$timeMocker->mock(function() {
    return '123456';
});
// string(6) "123456" int(1) 
var_dump(time(), $timeMocker->getNumberOfCalls());