I have the following code

    public function openConnection()
    {
        $this->ldapServerHandle = ldap_connect(ConfigH::getConfig()->ldap->host);

        $bindDN = ConfigH::getConfig()->ldap->serviceAccount->bindDN;

        if ($this->ldapServerHandle) {
            $this->ldapBindHandle = ldap_bind(
                $this->ldapServerHandle,
                $bindDN,
                ConfigH::getConfig()->ldap->serviceAccount->password
            );
            if (!$this->ldapBindHandle) {
                $errorMsg = "LDAP::__construct(): Could not bind the service account ".$bindDN;
                LoggerH::emergency($errorMsg);
                throw new LDAPException($errorMsg);
            }
        } else {
            $errorMsg = "LDAP::__construct(): Could not connect to the LDAP server ".ConfigH::getConfig()->ldap->host;
            LoggerH::emergency($errorMsg);
            throw new LDAPException($errorMsg);
        }
    }

The issue

I have this error causing me headaches since this morning:
Warning: ldap_bind(): Unable to bind to server: Can't contact LDAP server [...]

Everything worked fine on Windows, when I executed the code on our CentOS production server, it stopped working.

What I have already checked

• OpenLDAP is installed and enabled
• The LDAP server is reachable from the CentOS server (Kerberos is experiencing no issues on the same machine)

What I have already tried

• Re-installed the php-ldap extension
• Checked the credentials and the address a milion times

Additional information

• ConfigH::getConfig()->ldap->host returns something like "adserver.ourcompany.com", which is the address of our LDAP server
• ConfigH::getConfig()->ldap->serviceAccount->bindDN returns a valid bind DN
• ConfigH::getConfig()->ldap->serviceAccount->password returns the password of the service account