Whenever you want to use another script inside php code you need to use the <<< command. In this case you do this way:

<?php
if ( !defined('BLOCK_FILE') ) {
    Header('Location: ../index.php');
    die();
}

$content = <<<_JSCODE_
<script>T.searchbox()</script>
_JSCODE_;
?>

Note that after _JSCODE_ must be no spaces.