MySQLi Dynamic Prepared Statements strange behavior - php

Question

So I have successfully wrote a script that will prepare and bind any parameters to a sql query but I am having trouble on the binding results part.
Here is my script:
public function query( $query_string, $params = null, $param_types = null) {
//prepare the statement
$stmt = $this->db->prepare($query_string);
//check if the sql query is wrong.
if($stmt === false) {
echo "Wrong SQL: " . $query_string . "<br />Error: " . $this->db->errno . " " . $this->db->error;
}
if($params != null && $param_types != null) {
//build the array that needs to pass the args to bind_param.
$a_params = array();
$a_params[] = &$param_types;
for($i=0; $i<count($params); $i++)
$a_params[] = &$params[$i];
// $stmt->bind_param('s', $param); equivalent
call_user_func_array(array($stmt, 'bind_param'), $a_params);
}
//run the query
$stmt->execute();
$data = $stmt->result_metadata();
$fields = array();
$out = array();
$count = 0;
while($field = $data->fetch_field()) {
$fields[$count++] = &$out[$field->name];
}
call_user_func_array(array($stmt, 'bind_result'), $fields);
$results = array();
$k = 0;
// loop through all result rows
while ($stmt->fetch()) {
$results[$k++] = $out;
print_r($out);
echo "<br />";
}
$stmt->close();
return $results;
}
It seems to bind correctly when I output with print_r, but when I add to an array to return (for use in later script it has odd behavior).
Example call to the method:
$users = $TCS->query("SELECT name, age, id FROM test");
foreach($users as $user) {
echo $user['name'] . ': <br />';
echo ' age: ' . $user['age'] . '<br />';
echo ' id: ' . $user['id'] . '<br />';
}
But here is my output:
Array ( [name] => jake [age] => 18 [id] => 1 )
Array ( [name] => ryan [age] => 19 [id] => 2 )
Array ( [name] => stephen [age] => 16 [id] => 3 )
stephen:
age: 16
id: 3
stephen:
age: 16
id: 3
stephen:
age: 16
id: 3

Answer 1

TL;DR
Change $results[$k++] = $out; to $results[$k++] = array_flip(array_flip($out));
Here is an explanation of what is happening.
var_dump($out);
// first iteration
array(2) {
["name"]=>
&string(4) "mike"
["id"]=>
&int(1)
}
// second interation
array(2) {
["name"]=>
&string(6) "amanda"
["id"]=>
&int(2)
}
The important point to notice here are the ampersands, they mean that the values of id and name are references. What those references are pointing to will be changed when $out['id'] and $out['name'] are. Thus the statement $results[$k++] = $out; means copy $out and assign it to $results[$k], this includes copying the references. In the next iteration what is being referenced in $results[$k-1] is changed to the new values behind your back.
Here is a simple example;
$i = 1;
$a = array('id' => &$i);
$b = $a;
$i = 2;
var_dump($a,$b);
// output
array(1) {
["id"]=>
&int(2)
}
array(1) {
["id"]=>
&int(2)
}
You can fix this by modifying the line:
$results[$k++] = $out; to $results[$k++] = array_flip(array_flip($out));
The inner array_flip will dereference the values and make them keys, the outer flip reverse the process.
But I would suggest rewriting everything below $stmt->execute() as follows.
//run the query
$stmt->execute();
$result = $stmt->get_result();
if (!$result) { return array(); }
$ret = array();
while ($row = $result->fetch_assoc()) {
array_push($ret, $row);
}
return $ret;