PHP 8.x: various compatibility fixes for WordPress 6.8

[desrosj]

Previously:

• #59654 (6.7)
• #57837 (6.3)
• #56790 (6.2)
• #55656 (6.1)
• #54730 (6.0)
• #53635 (5.9)
• #50913 (5.6)

This ticket will be used as an "epic", allowing a variety of small patches each fixing a specific failure to be added to and committed against this ticket.

For patches addressing all instances of failures related to one specific PHP version (such as PHP 8.0, 8.1, or 8.2) change across the codebase, separate tickets should still be opened.

For an example of issues/patches with separate tickets, see:

• #53299 PHP 8.1: Update is_serialized function to accept Enums
• #53465 PHP 8.1.: the default value of the flags parameter for htmlentities() et all needs to be explicitly set

When opening a separate ticket, please tag it with the appropriate PHP version keyword so that these tickets can be easily found:

• PHP 8.4 keyword is php84 with its report https://core.trac.wordpress.org/query?keywords=~php84
• PHP 8.3: keyword is php83 with its report https://core.trac.wordpress.org/query?keywords=~php83
• PHP 8.2: keyword is php82 with its report https://core.trac.wordpress.org/query?keywords=~php82
• PHP 8.1: keyword is php81 with its report https://core.trac.wordpress.org/query?keywords=~php81
• PHP 8.0: keyword is php8 with its report https://core.trac.wordpress.org/query?keywords=~php8