Collaboration: Define WP_ALLOW_COLLABORATION inline

[adamziel]

What it does

Moves the WP_ALLOW_COLLABORATION constant definition out of wp_functionality_constants() in default-constants.php and into a new wp_is_collaboration_allowed() function in collaboration.php. The constant is now defined on first access rather than at a fixed point in the boot sequence.

Rationale

wp_functionality_constants() runs in wp-settings.php after collaboration.php is loaded. If anything calls wp_is_collaboration_enabled() before that point — for example during a SHORTINIT boot — the constant doesn't exist yet.

The same gotcha applies to AUTOSAVE_INTERVAL, EMPTY_TRASH_DAYS, and other constants in that function, but collaboration is new code, so we can avoid the pattern from the start.

Implementation

Added wp_is_collaboration_allowed() which:

1. Checks if WP_ALLOW_COLLABORATION is already defined (e.g. in wp-config.php)
2. If not, reads the WP_ALLOW_COLLABORATION environment variable
3. Defines the constant — defaulting to true if no env var is set, or 'true' === $env_value if it is

wp_is_collaboration_enabled() now delegates to wp_is_collaboration_allowed() instead of checking the constant directly. The admin UI in options-writing.php calls wp_is_collaboration_allowed() to decide whether to render the checkbox or the "disabled" notice.

Testing instructions

1. Verify the Writing Settings page shows the collaboration checkbox when WP_ALLOW_COLLABORATION is not defined or is true
2. Add define( 'WP_ALLOW_COLLABORATION', false ); to wp-config.php — the checkbox should be replaced by a "disabled" notice
3. Remove the constant and set WP_ALLOW_COLLABORATION=false as an environment variable — same "disabled" notice should appear

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Props audrasjb, desrosj, sergeybiryukov, joedolson, johnbillion, wildworks, dmsnell, ellatrix, jonsurrell, mcsf, jorgefilipecosta, johnjamesjacoby, westonruter, flixos90, peterwilsoncc, adamsilverstein, gziolo.

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