Assigments may be split onto several lines when the character/line limit would be exceeded. The equal sign has to be positioned onto the following line, and indented by 4 characters.
<?php
$GLOBALS['TSFE']->additionalHeaderData[$this->strApplicationName]
= $this->xajax->getJavascript(t3lib_extMgm::siteRelPath('nr_xajax'));
?>