How to check for the presence of a string when scanning ClamAV from its signatures?

Question

Information Security

The question is how to scan ClamAV not by the full sample of the file, but by the presence of a specific code? For example, there is a file with a size of 494 bytes. It is added to the signature database by MD5 hash and size "a4cb4d606c77da0aa9c687e378bb89ee:494:global.php" With this type of addition, there are files that contain only this code, if we add a couple of characters, there will be no triggering.