almost certainly is malicious, and there are several risks introduced by the code provided.
The first and loudest sign of trouble here is going to be invocation of the
eval() command. When the input is constructed from
$_COOKIE—a superglobal which effectively allows for anonymous clients to store whatever short strings they want to via any HTTP method—then straight away you are opened up to arbitrary code execution. To make matters worse, the code uses the
@ symbol in that invocation to suppress error messages or any output that may otherwise arise, thus preventing you from reviewing what has run and what will be run later.
Basically, it looks like an attacker has set it up to pass in arbitrary code through cookies. The bad actor here could be doing any number of things—on the more benign end, they may be showing ads that aren't yours or redirecting to their own site for traffic gains, or on the more intense side could be camping on your server and periodically stealing all of your users' data from the users directly as well as any connected data stores. You should be taking serious measures to address this in the event that you have customer data present and accessible from this server. It's a leak.
Consider pursuing another host if the provider you are with is not able or willing to assist you in diagnosing the origin here. It possibly came from some dependency in your code and may be replicated via your git configuration since you mention that it is recurring, so consider an audit of your repositories and that configuration. Maybe switch to a host like Cloudflare, often free.
Since this is PHP/MySQL, you would perhaps be better served by other providers. I was just throwing out one name, but other trusted providers like AWS/Azure/Google Cloud will have what you want. In general, Digital Ocean has quite a low barrier to entry.
To prevent a repeat attack, as noted by OscarGarcia: If you uploaded a private key anywhere on this server, or anywhere at all in raw form—be that a shared symmetric key or the private half of a asymmetric key pair—then you should go ahead and retire that right away.
This attack would have granted access to the host file system and, depending on what the key is used for, may leave you vulnerable to a repeat attack elsewhere.