Symfony/PHP: ldap_explode_dn function problem

I use the ldap_explode_dn Function for encoding my ldap string.

$ldapDnParts = ldap_explode_dn($groupMembership, 1);

But I get the Undefined-Function-Error:

Attempted to call function "ldap_explode_dn" from namespace "AppFactory".

The class where I call teh function is under AppFactory. I do not understand why it takes this namespace when searching the native function.
Can anyone help me with this?
I use PHP8 and Symfony 5.4.