PHP server optimization

We have a server which receives users requests, call third parties API, get response and notifies users. We are experiancing PHP crushes in case the third party API respond slowly (Many pending requests). We are lloking then a expert to see logs and cause of those PHP down and do necessary so that PHP can still working even in case of many pending requests waiting response from third party API. The server is Centos 7 and use Mysql MariadB database. Mysql still working fine when PHP goes down.
I want you you describe how you caount to proceed. Not serious refrain.