imagewebp() returns true when encoding fails #24457

Description

The following code:

$im = imagecreate(4, 4);
var_dump(imagewebp($im, 'out.webp'));

Resulted in this output:

Warning: imagewebp(): Palette image not supported by webp
bool(true)

(out.webp is empty.)

But I expected this output instead:

Warning: imagewebp(): Palette image not supported by webp
bool(false)

The same happens when libwebp rejects the image, e.g. imagecreatetruecolor(16384, 1): a warning, then true. This comes from gdImageWebpCtx() returning void.

PHP Version

PHP 8.5.13-dev (cli)