Get domain "referrer" name on instances that behind Load Balancer

Dear all,

I have setup load balancer and 2 instance behind it, and we make a services that client can put like external script on their site and call my script…so like call https://mydomain.com/call/function, so like this scenario

on website client https://clientsite.com inside the html call “https://mydomain.com/call/function” to get the data from my services…but i need domain name which is “clientsite.com” on my php script on server…but the server is behind the load balancer which is what i got is the IP from load balancer to instance / server…

but what i want or need is get the domain name of the domain that access or call my function…because from access log i just can see all same ip which is i believe from load balancer (what i want is the real domain that access my server)

is there is any tutorial for this? thank you so much before…sorry if my english hard to understand…thank you again!