Yes, you need a codec for libavif.  Try installing libaom.

I think we need to do checks for encoding and decoding in
config.m4.

I tried recompiling gd on Debian Buster after adding libaom:

apt-get install -y libfreetype6-dev libjpeg62-turbo-dev libpng-dev libxpm-dev libwebp-dev libavif-dev libaom-dev

But

php -r 'var_dump(imageavif(imagecreatetruecolor(8, 8), "/tmp/test"));'

still gives a Segmentation fault



I also tried recompiling gd on Alpine Linux 3.14 after adding aom:

apk add freetype-dev libjpeg-turbo-dev libpng-dev libxpm-dev libwebp-dev libavif-dev aom-dev

but 

php -r 'var_dump(imageavif(imagecreatetruecolor(8, 8), "/tmp/test"));'

still outputs

Warning: imageavif(): avif error - Could not encode image: No codec available in Command line code on line 1
bool(true)

(and the generated file is empty)

Hmm, I wonder whether we need to explicitly configure a codec for
libavif on some platforms.  Apparently, pkg-config doesn't do this
here.

This issue is still live with version 8.1.0beta3

Same test procedure, but used docker image php:8.1.0beta3-fpm-buster

I think this is the same thing that Pierre kindly fixed here:

https://github.com/libgd/libgd/issues/770

I filed MR to upgrade libavif in Alpinelinex 3.14 but looks it needs major version bump for libaom, meantime it should be working in alpine:edge

So, it seems that the root cause of this issue is that the libaom versions currently provided by Debian APT and Alpine APK systems are not functional (AFAIK they are compiled without the required BUILD_SHARED_LIBS=1, but I may be wrong here).

So, in order to make imageavif() and imagecreatefromavif() work, you need to do everything by hand:

1. compile and install libaom
2. compile and install libdav1d (to decode AV1)
3. compile and install libyuv (used by libavif to better handle YUV decoding)
4. compile and install libavif
5. compile and install the gd PHP extension (with the --with-avif flag)

For an example about how that can be done in Debian (so, Ubuntu too) and Alpine, you can take a look at https://github.com/mlocati/docker-php-extension-installer

> So, it seems that the root cause of this issue is that the
> libaom versions currently provided by Debian APT and Alpine APK
> systems are not functional […]

Right.  Closing as not a (PHP) bug.

Just an update on this issue.

Alpine Linux offers the pre-compiled libraries avif/aom/dav1d.

With Alpine up to 3.14, by using these libraries we had this error:

imageavif(): avif error - Could not encode image: No codec available


But since Alpine 3.15 and later, it seems we can use them without problems, that is:

apk add libavif-dev aom-dev dav1d-dev

Update: on Debian 12 (Bookworm) we can use the system-provided libraries:

apt-get install libavif-dev libaom-dev libdav1d-dev

This can't be done on previous Debian versions: invoking imageavif() was leading to this error

Error 2: imageavif(): avif error - Could not encode image: No codec available