public function testZeroFill()
 {
     $this->assertEquals('0000', StringUtils::zeroFill(''));
     $this->assertEquals('0001', StringUtils::zeroFill('1'));
     $this->assertEquals('0010', StringUtils::zeroFill(10));
     $this->assertEquals('123123', StringUtils::zeroFill('123123'));
     $this->assertEquals('00ab', StringUtils::zeroFill('ab'));
     $this->assertEquals('000ab', StringUtils::zeroFill('ab', 5));
     $this->assertEquals('000000123456', StringUtils::zeroFill('123456', 12));
 }