src/Zend/Filter/RealPath.php
public function filter($value)
    {
        $path = (string) $value;
        if ($this->_exists) {
            return realpath($path);
        }

        $realpath = @realpath($path);
        if ($realpath) {
            return $realpath;
        }

        $drive = '';
        if (substr(PHP_OS, 0, 3) == 'WIN') {
            $path = preg_replace('/[\\\\\/]/', DIRECTORY_SEPARATOR, $path);
            if (preg_match('/([a-zA-Z]\:)(.*)/', $path, $matches)) {
                list($fullMatch, $drive, $path) = $matches;
            } else {
                $cwd   = getcwd();
                $drive = substr($cwd, 0, 2);
                if (substr($path, 0, 1) != DIRECTORY_SEPARATOR) {
                    $path = substr($cwd, 3) . DIRECTORY_SEPARATOR . $path;
                }
            }
        } elseif (substr($path, 0, 1) != DIRECTORY_SEPARATOR) {
            $path = getcwd() . DIRECTORY_SEPARATOR . $path;
        }

        $stack = array();
        $parts = explode(DIRECTORY_SEPARATOR, $path);
        foreach ($parts as $dir) {
            if (strlen($dir) && $dir !== '.') {
                if ($dir == '..') {
                    array_pop($stack);
                } else {
                    array_push($stack, $dir);
                }
            }
        }

        return $drive . DIRECTORY_SEPARATOR . implode(DIRECTORY_SEPARATOR, $stack);
    }
public function filter($value)
    {
        $path = (string) $value;
        if ($this->_exists) {
            return realpath($path);
        }

        $realpath = @realpath($path);
        if ($realpath) {
            return $realpath;
        }

        $drive = '';
        if (substr(PHP_OS, 0, 3) == 'WIN') {
            $path = preg_replace('/[\\\\\/]/', DIRECTORY_SEPARATOR, $path);
            if (preg_match('/([a-zA-Z]\:)(.*)/', $path, $matches)) {
                list($fullMatch, $drive, $path) = $matches;
            } else {
                $cwd   = getcwd();
                $drive = substr($cwd, 0, 2);
                if (substr($path, 0, 1) != DIRECTORY_SEPARATOR) {
                    $path = substr($cwd, 3) . DIRECTORY_SEPARATOR . $path;
                }
            }
        } elseif (substr($path, 0, 1) != DIRECTORY_SEPARATOR) {
            $path = getcwd() . DIRECTORY_SEPARATOR . $path;
        }

        $stack = array();
        $parts = explode(DIRECTORY_SEPARATOR, $path);
        foreach ($parts as $dir) {
            if (strlen($dir) && $dir !== '.') {
                if ($dir == '..') {
                    array_pop($stack);
                } else {
                    array_push($stack, $dir);
                }
            }
        }

        return $drive . DIRECTORY_SEPARATOR . implode(DIRECTORY_SEPARATOR, $stack);
    }
Defined by Zend_Filter_Interface

Returns realpath($value)

@param  string $value
@return string