Having trouble with a relationship query with the SalesForce API

SoapFault Object
(
    [message:protected] => INVALID_TYPE: 
t2.TrackingNumber__c FROM OpportunityLineItem t1, Opportunity t2
                                                  ^
ERROR at Row:1:Column:74
A driving SObject type has already been set, all other entity types in the FROM clause must be relationships to the initial object.  The driving object is OpportunityLineItem.
    [string:private] => 
    [code:protected] => 0
    [file:protected] => C:\xampp\htdocs\SALESFORCE INTEGRATION\phptoolkit-13_0\soapclient\SforceBaseClient.php
    [line:protected] => 661
    [trace:private] => Array
        (
            [0] => Array
                (
                    [function] => __call
                    [class] => SoapClient
                    [type] => ->
                    [args] => Array
                        (
                            [0] => query
                            [1] => Array
                                (
                                    [0] => Array
                                        (
                                            [queryString] => SELECT t1.Description, t2.TrackingNumber__c FROM OpportunityLineItem t1, Opportunity t2
                                        )
 
                                )
 
                        )
 
                )
 
            [1] => Array
                (
                    [file] => C:\xampp\htdocs\SALESFORCE INTEGRATION\phptoolkit-13_0\soapclient\SforceBaseClient.php
                    [line] => 661
                    [function] => query
                    [class] => SoapClient
                    [type] => ->
                    [args] => Array
                        (
                            [0] => Array
                                (
                                    [queryString] => SELECT t1.Description, t2.TrackingNumber__c FROM OpportunityLineItem t1, Opportunity t2
                                )
 
                        )
 
                )
 
            [2] => Array
                (
                    [file] => C:\xampp\htdocs\SALESFORCE INTEGRATION\phptoolkit-13_0\soapclient\SforcePartnerClient.php
                    [line] => 155
                    [function] => query
                    [class] => SforceBaseClient
                    [type] => ->
                    [args] => Array
                        (
                            [0] => SELECT t1.Description, t2.TrackingNumber__c FROM OpportunityLineItem t1, Opportunity t2
                        )
 
                )
 
            [3] => Array
                (
                    [file] => C:\xampp\htdocs\SALESFORCE INTEGRATION\phptoolkit-13_0\sftest.php
                    [line] => 34
                    [function] => query
                    [class] => SforcePartnerClient
                    [type] => ->
                    [args] => Array
                        (
                            [0] => SELECT t1.Description, t2.TrackingNumber__c FROM OpportunityLineItem t1, Opportunity t2
                        )
 
                )
 
        )
 
    [faultstring] => INVALID_TYPE: 
t2.TrackingNumber__c FROM OpportunityLineItem t1, Opportunity t2
                                                  ^
ERROR at Row:1:Column:74
A driving SObject type has already been set, all other entity types in the FROM clause must be relationships to the initial object.  The driving object is OpportunityLineItem.
    [faultcode] => sf:INVALID_TYPE
    [detail] => stdClass Object
        (
            [InvalidSObjectFault] => SoapVar Object
                (
                    [enc_type] => 0
                    [enc_value] => stdClass Object
                        (
                            [exceptionCode] => INVALID_TYPE
                            [exceptionMessage] => 
t2.TrackingNumber__c FROM OpportunityLineItem t1, Opportunity t2
                                                  ^
ERROR at Row:1:Column:74
A driving SObject type has already been set, all other entity types in the FROM clause must be relationships to the initial object.  The driving object is OpportunityLineItem.
                            [row] => 1
                            [column] => 74
                        )
 
                    [enc_stype] => InvalidSObjectFault
                    [enc_ns] => urn:fault.partner.soap.sforce.com
                )
 
        )
 
)
                                              

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:
39:
40:
41:
42:
43:
44:
45:
46:
47:
48:
49:
50:
51:
52:
53:
54:
55:
56:
57:
58:
59:
60:
61:
62:
63:
64:
65:
66:
67:
68:
69:
70:
71:
72:
73:
74:
75:
76:
77:
78:
79:
80:
81:
82:
83:
84:
85:
86:
87:
88:
89:
90:
91:
92:
93:
94:
95:
96:
97:
98:
99:
100:
101:
102:
103:
104:
105:
106:
107:
108:
109:
110:
111:
112:

SObject Object ( [type] => OpportunityLineItem [fields] => stdClass Object ( [Description] => asdf [1] => 
SObject Object ( [type] => Opportunity [fields] => stdClass Object ( [TrackingNumber__c] => 123456 ) ) ) ) 

                                              

1:
2:

Notice: Undefined property: stdClass::$TrackingNumber__c in C:\xampp\htdocs\SALESFORCE INTEGRATION\phptoolkit-13_0\sftest.php on line 40
Tracking Number: 
Description: test
                                              

1:
2:
3: