You can finetune the key generation (such as specifying the number of bits) using configargs.
openssl_pkey_new() generates a new private and public key pair. The public component of the key can be obtained using openssl_pkey_get_public(). You can finetune the key generation (such as specifying the number of bits) using configargs. See openssl_csr_new() for more information about configargs.