I am currently in the process of replacing Mcrypt with OpenSSL since Mcrypt will be deprecated in PHP 7.1. I need is a way to get the blocksize per algorithm like mcrypt_get_block_size().
I am wondering if there is an equivalent function to mcrypt_get_block_size() but it's pretty badly documented can't seem to find it.
php-openssl unfortunately doesn't have an API that would give you the cipher blockSize. If you really need it, you'd have to hard-code the blockSize (per algorithm).
However, typical applications would only need to support a single encryption algorithm, and in that case you should already know what the block size is for your case.
And also, the only use cases I've had for mcrypt_get_block_size(), mcrypt_enc_get_block_size() is PKCS#7 padding, which OpenSSL already does by default for block cipher algorithms. So it may be the case that you don't need this at all.