You can try
$str = 'Foo
bar';
var_dump(replaceTag($str,array("h1"=>"div")));
Output
string 'Foo
bar' (length=17)
Function Used
function replaceTag($str,$tags) {
foreach ( $tags as $old => $new )
$str = preg_replace("~<(/)?$old>~", "<\\1$new>", $str);
return $str;
}