apply_filters( 'mce_buttons_2', array $mce_buttons_2, string $editor_id )
Filters the second-row list of TinyMCE buttons (Visual tab).
Parameters
- $mce_buttons_2
-  (array) Second-row list of buttons. 
- $editor_id
-  (string) Unique editor identifier, e.g. 'content'. Accepts 'classic-block' when called from block editor's Classic block. 
Source
Changelog
| Version | Description | 
|---|---|
| 3.3.0 | The $editor_idparameter was added. | 
| 2.0.0 | Introduced. | 
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Licensed under the GNU GPLv2+ License.
    https://developer.wordpress.org/reference/hooks/mce_buttons_2