Brightness Calc PHP

<?php

  function calc_brightness($color) {
    $rgb = hex2RGB($color);
    return sqrt(
       $rgb["red"] * $rgb["red"] * .299 +
       $rgb["green"] * $rgb["green"] * .587 +
       $rgb["blue"] * $rgb["blue"] * .114);
  }

  function render_spans() {
    $a = array();
    $a[] = "#ffff00";
    $a[] = "#336699";
    $a[] = "#ffffff";
    $a[] = "#000000";
    $a[] = "#666666";
    $a[] = "#ff00ff";
    $a[] = "#00ff00";

    foreach($a as $item) {
      $brightness = calc_brightness($item);
      $fore_color = ($brightness < 130) ? "#FFFFFF" : "#000000";
      echo '<span style="background:'.$item.'; color:'.$fore_color.'">Some Text!!!</span>';
    }
  }

  //http://www.php.net/manual/en/function.hexdec.php#99478
  function hex2RGB($hexStr, $returnAsString = false, $seperator = ',') {
      $hexStr = preg_replace("/[^0-9A-Fa-f]/", '', $hexStr); // Gets a proper hex string
      $rgbArray = array();
      if (strlen($hexStr) == 6) { //If a proper hex code, convert using bitwise operation. No overhead... faster
          $colorVal = hexdec($hexStr);
          $rgbArray['red'] = 0xFF & ($colorVal >> 0x10);
          $rgbArray['green'] = 0xFF & ($colorVal >> 0x8);
          $rgbArray['blue'] = 0xFF & $colorVal;
      } elseif (strlen($hexStr) == 3) { //if shorthand notation, need some string manipulations
          $rgbArray['red'] = hexdec(str_repeat(substr($hexStr, 0, 1), 2));
          $rgbArray['green'] = hexdec(str_repeat(substr($hexStr, 1, 1), 2));
          $rgbArray['blue'] = hexdec(str_repeat(substr($hexStr, 2, 1), 2));
      } else {
          return false; //Invalid hex color code
      }
      return $returnAsString ? implode($seperator, $rgbArray) : $rgbArray; // returns the rgb string or the associative array
  }

?>

<!doctype html>
<head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
  <title>Color Brightness PHP Test</title>
  <style type="text/css">
    #target_container span { padding:30px; color:#fff; display:block; width:100px; background:#000; float:left; margin:10px;}
  </style>
</head>
<body>
  <div id="target_container">
    <?php render_spans(); ?>
  </div>
</body>
</html>