WP_Theme should use get_file_data() for retrieving page templates

[dd32]

Currently WP_Theme::get_page_templates() uses a direct regular expression on the result of get_file_contents(), it should instead use the get_file_data() helper method we have to interact with file header data.

Currently switching will result in a back-compat break, as some themes have been using headers such as <?php // Template Name: Something ?> as the header, which get_file_data() doesn't like - See #33387

See r21117 for where we previously did use it temporarily during the 3.4 cycle, but broke due to #33387

[m_uysl]

depends on #33387

[Thomas Vitale]

Patch refreshed

[joyously]

This ticket proposes a "should", but says it is not backward compatible. So why "should" it be changed?
It's not broken, so why change it?

[SergeyBiryukov]

Replying to joyously:

This ticket proposes a "should", but says it is not backward compatible. So why "should" it be changed?

The backward compatibility concerns are addressed in [51182] / #33387, so this can now be addressed as well.

Why? Using an existing function instead of partially duplicating its functionality should make future maintenance easier.

[audrasjb]

As per today's bug scrub, let's move this ticket to the next milestone for proper review/refresh.

[costdev]

Patch refreshed against trunk. Added missing {} in if statements, removed extraneous semi-colon + PHPCS fixes.

@cosdev's patch https://core.trac.wordpress.org/ticket/33388

[peterwilsoncc]

I created a ​pull request based in 33888.3.diff​ to get the tests running but there are few errors and failures that will need a little investigation.

Closing this as it was just to get tests running.

[peterwilsoncc]

Per discussion in recent bug scrub, I'm moving this off 6.0.

Props @costdev, @chaion07