Are Dynamic Prepared Statements Bad? (with php + mysqli) - php

Question

I like the flexibility of Dynamic SQL and I like the security + improved performance of Prepared Statements. So what I really want is Dynamic Prepared Statements, which is troublesome to make because bind_param and bind_result accept "fixed" number of arguments. So I made use of an eval() statement to get around this problem. But I get the feeling this is a bad idea. Here's example code of what I mean
// array of WHERE conditions
$param = array('customer_id'=>1, 'qty'=>'2');
$stmt = $mysqli->stmt_init();
$types = ''; $bindParam = array(); $where = ''; $count = 0;
// build the dynamic sql and param bind conditions
foreach($param as $key=>$val)
{
$types .= 'i';
$bindParam[] = '$p'.$count.'=$param["'.$key.'"]';
$where .= "$key = ? AND ";
$count++;
}
// prepare the query -- SELECT * FROM t1 WHERE customer_id = ? AND qty = ?
$sql = "SELECT * FROM t1 WHERE ".substr($where, 0, strlen($where)-4);
$stmt->prepare($sql);
// assemble the bind_param command
$command = '$stmt->bind_param($types, '.implode(', ', $bindParam).');';
// evaluate the command -- $stmt->bind_param($types,$p0=$param["customer_id"],$p1=$param["qty"]);
eval($command);
Is that last eval() statement a bad idea? I tried to avoid code injection by encapsulating values behind the variable name $param.
Does anyone have an opinion or other suggestions? Are there issues I need to be aware of?

Answer 1

I think it is dangerous to use eval() here.
Try this:
iterate the params array to build the SQL string with question marks "SELECT * FROM t1 WHERE p1 = ? AND p2 = ?"
call prepare() on that
use call_user_func_array() to make the call to bind_param(), passing in the dynamic params array.
The code:
call_user_func_array(array($stmt, 'bind_param'), array($types)+$param);

Answer 2

I made a filter function which recives an array an asociative array like $_GET:
In model class I've defined a couple of properties including the schema:
private $table_name = "products";
protected $schema = [
'id' => 'INT',
'name' => 'STR',
'description' => 'STR',
'size' => 'STR',
'cost' => 'INT',
'active' => 'BOOL'
];
Then a filter method which recive an asociative arrays of conditions:
function filter($conditions)
{
$vars = array_keys($conditions);
$values = array_values($conditions);
$where = '';
foreach($vars as $ix => $var){
$where .= "$var = :$var AND ";
}
$where =trim(substr($where, 0, strrpos( $where, 'AND ')));
$q = "SELECT * FROM {$this->table_name} WHERE $where";
$st = $this->conn->prepare($q);
foreach($values as $ix => $val){
$st->bindValue(":{$vars[$ix]}", $val, constant("PDO::PARAM_{$this->schema[$vars[$ix]]}"));
}
$st->execute();
return $st->fetchAll(PDO::FETCH_ASSOC);
}
And works great to filter results

Answer 3

You don't really need prepared statements and bound arguments, because you can always use mysql_real_escape_string(). And you're right; dynamically generated SQL is far more flexible and valuable.
Here's a simple example using the regular mysql_* interface:
// Array of WHERE conditions
$conds = array("customer_id" => 1, "qty" => 2);
$wherec = array("1");
foreach ($conds as $col=>$val) $wherec[] = sprintf("`%s` = '%s'", $col, mysql_real_escape_string($val));
$result_set = mysql_query("SELECT * FROM t1 WHERE " . implode(" AND ", $wherec);
Of course, this is a simplistic example, and to make it useful you have to build and refine it a lot, but it shows the ideas and it's really very very useful. For example, here is a completely generic function to insert a new row into an arbitrary table, with the columns filled with the values from an associative array and completely SQL-injection safe:
function insert($table, $record) {
$cols = array();
$vals = array();
foreach (array_keys($record) as $col) $cols[] = sprintf("`%s`", $col);
foreach (array_values($record) as $val) $vals[] = sprintf("'%s'", mysql_real_escape_string($val));
mysql_query(sprintf("INSERT INTO `%s`(%s) VALUES(%s)", $table, implode(", ", $cols), implode(", ", $vals)));
}
// Use as follows:
insert("customer", array("customer_id" => 15, "qty" => 86));