EDIT: I've mixed and modified two of the answers given below to form the full function which now does what I had wanted and then some... So I figured I'd post it here in case anyone else comes looking for this same thing.

/*
 * Function to analyze string against many popular formatting styles of phone numbers
 * Also breaks phone number into it's respective components
 * 3-digit area code, 3-digit exchange code, 4-digit subscriber number
 * After which it validates the 10 digit US number against NANPA guidelines
*/
function validPhone($phone) {

  $format_pattern = '/^(?:(?:\((?=\d{3}\)))?(\d{3})(?:(?<=\(\d{3})\))?[\s.\/-]?)?(\d{3})[\s\.\/-]?(\d{4})\s?(?:(?:(?:(?:e|x|ex|ext)\.?\:?|extension\:?)\s?)(?=\d+)(\d+))?$/';
  $nanpa_pattern = '/^(?:1)?(?(?!(37|96))[2-9][0-8][0-9](?<!(11)))?[2-9][0-9]{2}(?<!(11))[0-9]{4}(?<!(555(01([0-9][0-9])|1212)))$/';

  //Set array of variables to false initially
  $valid = array(
    'format' => false,
    'nanpa' => false,
    'ext' => false,
    'all' => false
  );

  //Check data against the format analyzer
  if(preg_match($format_pattern, $phone, $matchset)) {
    $valid['format'] = true;    
  }

  //If formatted properly, continue
  if($valid['format']) {

    //Set array of new components
    $components = array(
      'ac' => $matchset[1], //area code
      'xc' => $matchset[2], //exchange code
      'sn' => $matchset[3], //subscriber number
      'xn' => $matchset[4], //extension number
    );

    //Set array of number variants
    $numbers = array(
      'original' => $matchset[0],
      'stripped' => substr(preg_replace('[\D]', '', $matchset[0]), 0, 10)
    );

    //Now let's check the first ten digits against NANPA standards
    if(preg_match($nanpa_pattern, $numbers['stripped'])) {
      $valid['nanpa'] = true;
    }

    //If the NANPA guidelines have been met, continue
    if($valid['nanpa']) {
      if(!empty($components['xn'])) {
        if(preg_match('/^[\d]{1,6}$/', $components['xn'])) {
          $valid['ext'] = true;
        }
      }
      else {
        $valid['ext'] = true;
      }
    }

    //If the extension number is valid or non-existent, continue
    if($valid['ext']) {
      $valid['all'] = true;
    }
  }
  return $valid['all'];
}