[Phpgroupware-developers] problem with ldap_modify

[kamaldeep kaur chawla]

hi,
i have a problem in interacting ldap in php.
i m unable to modify the attribute of a dn using ldap_mod_replace 
or ldap_modify.
following is code i followed:

$ds=ldap_connect("localhost");
$b=ldap_bind($ds,"cn=root,o=mars,c=in"," ");
$dn="cn=bal,o=mars,c=in";
$attr["userpassword"]="aaa";
$r=ldap_mod_replace($ds,$dn,$attr);
if($r===true)
{
echo "Password modified";
}

everytime i run the code i get the warning
modify operation could not be completed

please help me to find out the probelm.and yes i m also using 
ldap_search n ldap_comapre which r working fabuosly.
thanx
kamal