$t = preg_replace_callback(
    '#(<math>.*?</math>)#is', 
    function ($matches) {
        $t = str_replace(array('<math>','</math>'),'',$matches[0]); 
        $t = preg_replace_callback(
            '#(\$\$.*?\$\$)#is', 
            function ($matches) {
                return '£'.base64_encode($matches[0]).'£'; 
            },
            $t
        );
        $t = preg_replace_callback(
            '#(\$.*?\$)#is', 
            function ($matches) {
                return '£'.base64_encode($matches[0]).'£'; 
            },
            $t
        );
        return $t;
    },
    $t
);