Treat 'FALSE' as false in wp_validate_boolean()

[TobiasBg]

In [28542] for #28170, we introduced wp_validate_boolean() to allow converting the string 'false' into a boolean false.

Let's also allow uppercase ('FALSE') and mixed-case ('False') versions of the string, which could for example be coming from a Shortcode parameter, like [foo bar=FALSE /].

[kitchin]

From the patch:
if ( 'false' === strtolower( $var ) ) {

But strtolower() always returns a string so === is redundant. A non-scalar $var (like an object) might cause an error or warning. So I'd suggest:
if ( is_string( $var ) && 'false' == strtolower( $var ) ) {

[TobiasBg]

Nice catch, kitchin! Thanks!

30238.2.diff​ is a new patch with an is_string() check. I'd stick with the === check though, as it's cleaner and shows that we know that we are dealing with strings at this point.

[boonebgorges]

Oof. This whole function needs unit tests :)

[TobiasBg]

30238.2-tests.diff​ adds unit tests as well.

[boonebgorges]

In 30206:

Add unit tests for wp_validate_boolean().

Props TobiasBg.
See #30238.

[boonebgorges]

In 30207:

Ignore case when checking string 'false' in wp_validate_boolean().

Props TobiasBg, kitchin.
Fixes #30238.

[SergeyBiryukov]

In 47141:

Tests: Rename wpValidateBoolean.php for consistency with other files.

See #30238.