Funzione molto utile per formattare il testo XML in uscita (ad esempio da DomDocument).

function formatXmlString($xml, $indentBase = 0, $indentString = ' ') {
// add marker linefeeds to aid the pretty-tokeniser (adds a linefeed between all tag-end boundaries)
$xml = preg_replace('/(>)(<)(\/*)/', "$1\n$2$3", $xml);
// now indent the tags
$token = strtok($xml, "\n");
$result = ''; // holds formatted version as it is built
$pad = ($indentBase > 0) ? $indentBase : 0; // initial indent
$matches = array(); // returns from preg_matches()
// scan each line and adjust indent based on opening/closing tags
while ($token !== false):
// test for the various tag states
// 1. open and closing tags on same line - no change
if (preg_match('/.+<\/\w[^>]*>$/', $token, $matches)):
$indent = 0;
// 2. closing tag - outdent now
elseif (preg_match('/^<\/\w/', $token, $matches)):
$pad--;
// 3. opening tag - don't pad this one, only subsequent tags
elseif (preg_match('/^<\w[^>]*[^\/]>.*$/', $token, $matches)):
$indent = 1;
// 4. no indentation needed
else:
$indent = 0;
endif;
// pad the line with the required number of leading spaces
$line = str_pad($token, strlen($token) + $pad, $indentString, STR_PAD_LEFT);
$result .= $line . "\n"; // add to the cumulative result, with linefeed
$token = strtok("\n"); // get the next token
$pad += $indent; // update the pad size for subsequent lines
endwhile;
return $result;
}

Tratta (e modificata) da recurser.com.