Validate phone number

Chek if a phone numbers is valid, according to its syntax (should be: "+390523599314"). True if it's valid, False otherwise

function is_valid_phone( $phone ) 
{
   if ( $phone[0] != "+" ) {
      return false;
   } // end if
   else {
      $phone = substr( $phone, 1 ); // delete the "+"
      if ( !is_numeric( $phone ) ) {
         return false;
      } // end if
   } // end else
   return true;
} // end function is_valid_phone