Calculates and compares the MD5 of a file to its expected value.

function verify_file_md5( $filename, $expected_md5 ) {
	if ( 32 === strlen( $expected_md5 ) ) {
		$expected_raw_md5 = pack( 'H*', $expected_md5 );
	} elseif ( 24 === strlen( $expected_md5 ) ) {
		$expected_raw_md5 = base64_decode( $expected_md5 );
	} else {
		return false; // Unknown format.
	}

	$file_md5 = md5_file( $filename, true );

	if ( $file_md5 === $expected_raw_md5 ) {
		return true;
	}

	return new WP_Error(
		'md5_mismatch',
		sprintf(
			/* translators: 1: File checksum, 2: Expected checksum value. */
			__( 'The checksum of the file (%1$s) does not match the expected checksum value (%2$s).' ),
			bin2hex( $file_md5 ),
			bin2hex( $expected_raw_md5 )
		)
	);
}