"LDAP user and group backend" greyed out

Hello, I have set up Nextcloud, now i’m trying to activate "LDAP user and group backend"
The problem is when I try to activate, it is greyed out.

My nextcloud is running on a RHEL7 with php 5.6.32

Can you explain me why I can’t activate this ?

LDAP support in PHP?

Maybe because this option work with PHP LDAP extension ?

I close I will find help elsewhere.