Safety of AES ECB when used with openssl_seal function
ECB is not secure even with per file keys, because if two blocks of the file are identical, this is visible in the ciphertext. The only * cases where ECB is secure is encrypting completely random data or encrypting a single block per key.

You should pick something more secure if your can help it.

If there is literally no other option than RC4 and AES ECB, you could use first RC4 and then AES ECB. RC4 would make it almost random and AES ECB would hide the biases. However, general concerns with cascaded ciphers would apply.

* Ok, fine, or if plaintext blocks are known to be unique, but that's rare and not the case here.