You don't provide the table structure it's inserting into, but if you just want to solve the problem of splitting the $data array into its component parts, there are a few ways:

a) Use implode(), which should work fine despite you already mentioned trying it:

$data = [1,2,3];    
$sql = "INSERT INTO test_table VALUES (null,".implode(',',$data).")";

b) Quote each array index:

$data = [1,2,3];    
$sql = "INSERT INTO test_table VALUES (null,{$data[0]},{$data[1]},{$data[2]})";

But this only works if there is a fixed number of values ​​in the array.

c) Traverse the array:

$data = [1,2,3];    
$sql = "INSERT INTO test_table VALUES (null"
foreach($data as $value){ $sql .= ",$value"; }
$sql .= ")";

Hope this helps, if not please provide more details about the data being inserted and the database table structure so we can better understand the problem.