ConditionExpression attribute_not_exists() - php

Question

I am trying to use ConditionExpression when inserting an item on the database, but it dont work, the php script breaks when the Putitem() function runs.
I want to insert the item if he dont exist.
$response = $client->putItem(array(
'TableName' => 'tablename',
'Item' => array(
'serialNumber' => array('S' => 'test123'),
'deviceType' => array('S' => '1')
),
'ConditionExpression' => 'attribute_not_exists(serialNumber)'
));
I tried to var_dump the $response but the code breaks on the function above.
serialNumber its a Partition Key which should work as intended.
The code below works fine, but he replaces the existing item with new values, which its what i dont want to happen.
$response = $client->putItem(array(
'TableName' => 'tablename',
'Item' => array(
'serialNumber' => array('S' => 'test123'),
'deviceType' => array('S' => '1')
)
));

Answer 1

It is expected that you are returned a CondidtionCheckFailedException when the condition you set evaluates to false. Try wrapping your code in a try/catch block to see if it works as expected?
try {
$response = $client->putItem(array(
'TableName' => 'tablename',
'Item' => array(
'serialNumber' => array('S' => 'test123'),
'deviceType' => array('S' => '1')
)
));
}
catch(Exception $e) {
echo $e->getMessage();
}