PHP Tag replace glitch

Why does this not replace {{Title}} with jpp?
trk_fastswap( $pagedata, 'Title', 'jpp' );

echo $ret;

function trk_fastswap( &$haystack = '', $needle = 'Title', $replacement = 'jpp' )
{
    str_replace( '{{' . $needle . '}}', $replacement, $haystack );
}