[PHP-NOTES] note 130540 added to function.getimagesize

Looking at the number of channels, you can detect if the image SUPPORTS transparecy

function image_supports_transparency(string $file_path): ?bool {
	$ret = \getimagesize($file_path);
	$image_type = $ret[2];
	$supports_transparency = null;

	if (isset($ret['channels'])) {
		$channels = $ret['channles'];

		if ($image_type === \IMAGETYPE_JPEG || $image_type === \IMAGETYPE_GIF) {
			$supports_transparency = false;
		} elseif ($image_type === \IMAGETYPE_PNG) {
			if ($channels === 2 || $channels === 4) {
				$supports_transparency = true;
			} elseif ($channels !== 1) {
				// PNG also supports transparency via palette, which only uses a single channel.
				$supports_transparency = false;
			}
		} elseif ($image_type === \IMAGETYPE_WEBP) {
			$supports_transparency = $channels === 4;
		} elseif ($mime === 'image/avif') {
			$supports_transparency = $channels === 4 || $channels == 2;
		} elseif ($image_type === \IMAGETYPE_BMP) {
			$supports_transparency = $channels === 4;
		}
	}

	return $supports_transparency;
}

I say supports because an image may have an alpha channel but not actually use it, you'll need to check all the image's pixels to detect that.
Also note that it's not always possible to detect the alpha channel with just getimagesize(); see for example png which can define the number of channles actually used in the palette chunk.
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Manual Page -- https://php.net/manual/en/function.getimagesize.php
Edit        -- https://main.php.net/note/edit/130540
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