You can get the "innerHTML" by nodeValue so
<?php
$doc = new DOMDocument( );
$ele = $doc->createElement( 'p', 'Sensei Ninja' );
print $ele->nodeValue;
?>
You can even set it if you want
<?php
$doc = new DOMDocument( );
$ele = $doc->createElement( 'p' );
$ele->nodeValue = 'Sensei Ninja';
$doc->appendChild( $ele );
print $doc->saveHTML( );
?>