如何使用php在dynamodb中编写自定义查询？

In my case I have a table name "friends" which has two attributes:

• account_id
• fb_id

I try to get all the account_id based on fb_id, and for this wrote the query as below:

$response = $dynamoClient->getItem ( array(
            "TableName" => "facebook_friendlist",
            "ConsistentRead" => true,
            "Key" => array(
                "fb_id" => array(
                    "S" => "171092723281123"//$fbId 
                ) 
            ),
            "ProjectionExpression" => "account_id" 
        ) );

But its not getting executed. I also tried to write the same query using getItem(),scan(),getIterator() but no result.

Help me out with the right way. Thanks in advance.