src/Helpers/Html.php
public static function submitButton($content = 'Submit', $options = [], $encoding = null)
    {
        $options['type'] = 'submit';
        return static::button($content, $options, $encoding);
    }
public static function submitButton($content = 'Submit', $options = [], $encoding = null)
    {
        $options['type'] = 'submit';
        return static::button($content, $options, $encoding);
    }
Generates a submit button tag.

Be careful when naming form elements such as submit buttons. According to the [jQuery documentation](https://api.jquery.com/submit/) there
are some reserved names that can cause conflicts, e.g. `submit`, `length`, or `method`.

@param string $content the content enclosed within the button tag. It will NOT be HTML-encoded.
Therefore you can pass in HTML code such as an image tag. If this is is coming from end users,
you should consider [[encode()]] it to prevent XSS attacks.
@param array $options the tag options in terms of name-value pairs. These will be rendered as
the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
If a value is null, the corresponding attribute will not be rendered.
See [[renderTagAttributes()]] for details on how attributes are being rendered.
@param string $encoding
@return string the generated submit button tag