Error trying to bind invalid credentials - php

Question

I am trying to connect to LDAP server but I am always getting 'Error trying to bind: invalid credentials' when I supplied the right credentials. How do I troubleshoot this issue? How do I know if connection is successful? I am stuck here please let me know if there are any suggestions.
<?php
$domain = 'school123.edu';
$username = 'test';
$password = 'password';
$ldapconfig['host'] = 'server123';
$ldapconfig['port'] = 389;
$ldapconn=ldap_connect($ldapconfig['host'], $ldapconfig['port']);
ldap_set_option($ldapconn, LDAP_OPT_PROTOCOL_VERSION, 3);
ldap_set_option($ldapconn, LDAP_OPT_REFERRALS, 0);
// binding to ldap server
$ldapbind = ldap_bind($ldapconn, $username, $password) or die ("Error trying to bind: ".ldap_error($ldapconn));
// verify binding
if ($ldapbind) {
echo "LDAP bind successful...<br /><br />";
} else {
echo "LDAP bind failed...";
}
$dn='dc=school123,dc=edu';
$result = ldap_search($ldapbind,$dn,'(&(objectClass=*)(sAMAccountName=' . $username. '))');
$entries = ldap_get_entries($ldapconn, $result);
echo $entries["count"]." entries returned\n";
// all done? clean up
ldap_close($ldapconn);
?>