PHP LDAP验证证书 - ldap_start_tls（）：无法启动TLS

I am trying to do LDAP authenticate with certificate in Ubuntu

My PHP Code as follows :

   $ldap="ldap://myhostname";
   $usr="myemail@domain.com";
   $pwd="mypwd";

   $ds=ldap_connect($ldap);  
   $ldapbind=false;
   if(ldap_set_option($ds, LDAP_OPT_PROTOCOL_VERSION, 3))
      if(ldap_set_option($ds, LDAP_OPT_REFERRALS, 0)) 
         if(ldap_start_tls($ds)) 
               $ldapbind = @ldap_bind($ds, $usr, $pwd);    
   ldap_close($ds);

   if(!$ldapbind)
      echo "ERROR";
   else
      echo "OK";

In my ldap.conf file I added the following lines:

TLS_REQCERT never

TLS_CACERT /home/Desktop/rootCA.crt

But output as follows ldap_start_tls(): Unable to start TLS