Add subpage_file Filter Before Admin Menu

[Nessworthy]

For users extending the back end of wordpress, sometimes custom pages added to the main menu will not be flagged as the current page because of unexpected aspects such as additional GET data.

The problem is, setting this directly using a filter like admin_menu does not work due to $subpage_file being set later on (needs confirming), nor does it make sense due to context.

At the top of menu-header.php, there is a filter called parent_file, used to allow plugins/themes to move the submenu tabs around, a suggestion would be to add a subpage_file filter, allowing plugins and the theme to modify the compared url to check the current page.

Would add a patch, but away from my development environment.

[swissspidy]

24531.diff​ adds a submenu_file filter right after the parent_file filter. Not sure if $parent_file should be added as a second parameter, but this filter would be already very useful in my ​Admin Menu Manager plugin.

I think that could go into 4.4 with no problems.

[SergeyBiryukov]

Let's add $parent_file as a second argument.

[kraftbj]

adds parent_file arg

[kraftbj]

inline docs. Sorry Drew :(

[chriscct7]

Looks good. Nice work @swissspidy

[Nessworthy]

Happy to see it's all full steam ahead with this ticket!

[SergeyBiryukov]

In 34722:

Add 'submenu_file' filter for the file of an admin menu sub-menu item.

This complements the 'parent_file' filter added in [12712].

Props swissspidy, kraftbj.
Fixes #24531.

[DrewAPicture]

The $submenu_file global needs to be declared to avoid a notice here.

[DrewAPicture]

In 34724:

Declare the $submenu_file global to avoid a notice generated by passing it as a parameter to the submenu_file hook, introduced in [34722].

Fixes #24531.

[DrewAPicture]

In 34725:

Docs: Document declaration of the $submenu_file global in wp-admin/menu-header.php.

See #24531. See #32246.