Iam able to insert the data into object. But iam unable to update the same record using the external id . when i use upsert dml without external id (phpId) i can able to insert the data into custom object . when i use upsert with external id i am not able to insert the data into object . please help where i did worng

Product__c product= createProduct(params);
              
                if(product!=null){
                 body = upsertSObject(product);
                              
                if (body.id != null) {  
                   String name1 =null;
                   String value1 =null;  
                   String phpId=null;               
                   JSONParser parser = JSON.createParser(params.get('attributeValues',false));       
                  
                    while (parser.nextToken() != null) {
                          if ((parser.getCurrentToken() == JSONToken.FIELD_NAME) && (parser.getText() == 'name1')) {
                                 parser.nextToken();
                                 name1= parser.getText();
                                }
                          if ((parser.getCurrentToken() == JSONToken.FIELD_NAME) && (parser.getText() == 'value1')) {
                                parser.nextToken();
                                value1 = parser.getText();
                              }
                              if ((parser.getCurrentToken() == JSONToken.FIELD_NAME) && (parser.getText() == 'phpId')) {
                                parser.nextToken();
                                phpId= parser.getText();
                              }
                          if(name1 !=null && value1 !=null&&phpId !=null) {
                                Attribute__c atrb= createAttributeItem(name1,value1,phpId, body.id);
                                upsertSObject (atrb) ;
                                value1 = null;
                                phpId=null;
                               }
................


public static Attribute__c createAttributeItem(String name1,String value1,string phpId, Id id) {
   
        Attribute__c atb= new Attribute__c ();
        atb.Product_Name__c= id ;
        atb.Attribute_Value__c= value1;
        atb.Php_Id__c = phpId;
        atb.name = name1;
            
        return atb;
    }

..............

private static ResponseBody upsertSObject(SObject sObj) {
        System.debug(JSON.serializePretty(sObj));
        ResponseBody body = new ResponseBody();
        try {
            if (sObj instanceof Product__c)
          
              upsert (Product__c) sObj Php_External_Id__c ;
             else if(sObj instanceof Attribute__c )
           upsert (Attribute__c) sObj ;//here iam inserting attributes but if upsert (Attribute__c) sObj Php_Id__c iam not able to insert the data into object please       help    me  how to insert and update using external id here.
            body.id = sObj.id;
          }
        catch (Exception e) {
            body.addError(e);
          }
        return body;
    }