Memory management in php (laravel)

I have a Laravel 5.5 project running on php 7 on AWS ec2 instance with 1 gigabyte ram, and i have an api retrieve list of items after some http requests on external services.

After each request, i check the memory status using free -m command and the used memory increasing about 100 megabyte, and after 5 request i got cannot allocate memory, and it never free the memory, any help please?

The memory

via Chebli Mohamed