php camelcase to snake case

function from_camel_case($input) {
  $pattern = '!([A-Z][A-Z0-9]*(?=$|[A-Z][a-z0-9])|[A-Za-z][a-z0-9]+)!';
  preg_match_all($pattern, $input, $matches);
  $ret = $matches[0];
  foreach ($ret as &$match) {
    $match = $match == strtoupper($match) ?
      	strtolower($match) :
    	lcfirst($match);
  }
  return implode('_', $ret);
}

// Tests:
foreach ([
  'simpleTest' => 'simple_test',
  'easy' => 'easy',
  'HTML' => 'html',
  'simpleXML' => 'simple_xml',
  'PDFLoad' => 'pdf_load',
  'startMIDDLELast' => 'start_middle_last',
  'AString' => 'a_string',
  'Some4Numbers234' => 'some4_numbers234',
  'TEST123String' => 'test123_string',
] as $test => $result) {
  $output = from_camel_case($test);
  if ($output === $result) {
    echo "Pass: $test => $result\n";
  } else {
    echo "Fail: $test => $result [$output]\n";
  }
}
/*
Pass: simpleTest => simple_test
Pass: easy => easy
Pass: HTML => html
Pass: simpleXML => simple_xml
Pass: PDFLoad => pdf_load
Pass: startMIDDLELast => start_middle_last
Pass: AString => a_string
Pass: Some4Numbers234 => some4_numbers234
Pass: TEST123String => test123_string
*/