lib/PHPExif/Mapper/Native.php
protected function isFieldKnown(&$field)
    {
        $lcfField = lcfirst($field);
        if (array_key_exists($lcfField, $this->map)) {
            $field = $lcfField;

            return true;
        }

        $ucfField = ucfirst($field);
        if (array_key_exists($ucfField, $this->map)) {
            $field = $ucfField;

            return true;
        }

        return false;
    }
protected function isFieldKnown(&$field)
    {
        $lcfField = lcfirst($field);
        if (array_key_exists($lcfField, $this->map)) {
            $field = $lcfField;

            return true;
        }

        $ucfField = ucfirst($field);
        if (array_key_exists($ucfField, $this->map)) {
            $field = $ucfField;

            return true;
        }

        return false;
    }
Determines if the given field is known,
in a case insensitive way for its first letter.
Also update $field to keep it valid against the known fields.

@param  string  &$field
@return bool