If I leave out the wp_die() in the testiframe function I get the '0' appended to my output. If I put it in, the page w/ iframe linked to it crashes

add_action('wp_ajax_ajtestiframe','testiframe');

?php
function testiframe() {
?
    head
    style
    .aatest1 {color:red;}
    /style
    /head
    span class=aatest1This is a test/spanbr
?php
    echo 'This is my iframe contentbr';
    var_dump($_GET);
}
    wp_die();
?