code: public static function startsWith($string, $with, $caseSensitive = true)
      {
          if (!$bytes = static::byteLength($with)) {
              return true;
          }
          if ($caseSensitive) {
              return strncmp($string, $with, $bytes) === 0;
  
          }
          $encoding = Yii::$app ? Yii::$app->charset : 'UTF-8';
          return mb_strtolower(mb_substr($string, 0, $bytes, '8bit'), $encoding) === mb_strtolower($with, $encoding);
      }
docs: Check if given string starts with specified substring.
  Binary and multibyte safe.
  
  @param string $string Input string
  @param string $with Part to search inside the $string
  @param bool $caseSensitive Case sensitive search. Default is true. When case sensitive is enabled, $with must exactly match the starting of the string in order to get a true value.
  @return bool Returns true if first input starts with second input, false otherwise