Never going to happen unless you do it yourself. 20.04 will stick to 7.4 with security updates. From StableReleaseUpdates:
Once an Ubuntu release has been completed and published, updates for it are only released under certain circumstances, and must follow a special procedure called a "stable release update" or SRU.
Users of the official release, in contrast, expect a high degree of stability. They use their Ubuntu system for their day-to-day work, and problems they experience with it can be extremely disruptive. Many of them are less experienced with Ubuntu and with Linux, and expect a reliable system which does not require their intervention.
Stable release updates are automatically recommended to a very large number of users, and so it is critically important to treat them with great caution. Therefore, when updates are proposed, they must be accompanied by a strong rationale and present a low risk of regressions.
"It's just a one-line change!"
Even the simplest of changes can cause unexpected regressions due to lurking problems:
Stable release updates will, in general, only be issued in order to fix high-impact bugs. Examples of such bugs include:
See the link for more on this.
The packages list shows:
bionic (18.04LTS) (php): 1:7.2+60ubuntu1: all
focal (20.04LTS) (php): 2:7.4+75: all
impish (21.10) (php): 2:8.0+82~0build1: all
jammy (22.04LTS) (php): 2:8.1+92ubuntu1: all
kinetic (php): 2:8.1+92ubuntu1: all
The version is shown behind the
: so for 8.0 you need at least 22.04 (assuming LTS only)
You can install php8 yourself if you really want it but I would advice upgrading to 22.04.
If you want to be independent of php used in Ubuntu you can also install a tarball in
/opt and have that migrate through different versions (you can have a setup like
/opt/php/ and put a symlink in
/usr/bin/ that points to
/opt/php and that one holds a symlink to the active php version in the 2 other directories.