Format UK Phone Number

format-phone.php

<?php

/*
    $original = '+44 (0)1234 567 890';
    $original = '0044 01234 567 890';
    $original = '01234 567 890';
    $original = '44 1234 567 890';

    Result should always be:
    '+441234567890
*/

// Strip out all characters apart from numbers
$phone = preg_replace('/[^0-9]+/', '', $original);

// Remove the 2 digit international code (+44)
if (substr($phone, 0, 2) == '44') {
	$phone = substr($phone, 2);
}

// Remove the 4 digit international code (0044)
if (substr($phone, 0, 4) == '0044') {
	$phone = substr($phone, 4);
}

// Remove the initial Zero from the number
// Some people write it in international numbers like this: +44 (0)1234 567 890
// But it shouldn't be entered when dialling
if (substr($phone, 0, 1) == '0') {
	$phone = substr($phone, 1);
}

// Add the international prefix
$phone = '+44' . $phone;

Hope this might help, i've tweaked it a little:

function format_uk_phone($original) {
  /*
      $original = '+44 (0)1234 567 890';
      $original = '0044 01234 567 890';
      $original = '01234 567 890';
      $original = '44 1234 567 890';
      Result should always be:
      '+441234567890
  */

  // Strip out all characters apart from numbers
  $phone = preg_replace( '/[^0-9]+/', '', $original );

  if ( substr( $phone, 0, 2 ) == '00' ) {
    // Remove the international code (00)
    $phone = substr( $phone, 2 );
  }  
  if ( substr( $phone, 0, 2 ) == '44' ) {
    // Remove the international code (44)
    $phone = substr( $phone, 2 );
  }
  if ( substr( $phone, 0, 1 ) == '0' ) {
    // Remove the initial Zero from the number
    $phone = substr( $phone, 1 );
  } 
  if ( in_array(substr( $phone, 0, 1 ), array('1','2','7') ) && (strlen($phone) <= 10) ) {
    // Some people write it in international numbers like this: +44 (0)1234 567 890
    // But it shouldn't be entered when dialling
    // This is probably a UK number
  } else {
    // Phone is not a UK number
    $phone = '';
  }

  if ( $phone !== '' ) {
    // Add the international prefix
    $phone = '+44' . $phone;
  }
  return $phone;
}

Thanks @lukearmstrong