How can I avoid mistakes using the same variable name again?

Question

This isn't a rare case and occurs with me often and I spent countless time trying to debug the code.

Latest example, this PHP code where I used $email for the parameter and for object as well.

private function _mail( $email )
{
    if( filter_var( $email, FILTER_VALIDATE_EMAIL ) ):
        $email = new Zend_Mail();
        $email->setSubject( $this->request->post('subject') )
            ->setBodyHtml( $this->request->post('message') )
            ->setFrom( $this->request->post('from') )
            ->addTo( $email )
            ->send();
        unset($email);
    endif;
}

It was throwing an error, strtr() expects parameter 1 to be an string object given and it was really frustrating to browse through zend libraries to see what obscure string statement was throwing this error.

How to avoid such mistakes?