Error with bindParam overwriting in PHP

Question

This is a bit of a weird one, and I could well be coding this completely wrong - hence why I've hit the same error twice in two days, in completely different parts of a script. The code I'm using is below:

    public function findAll( $constraints = array() ) {

        // Select all records
        $SQL = 'SELECT * FROM ' . $this->tableName;

        // See if there's any constraints
        if( count( $constraints ) > 0 ) {
            $SQL .= ' WHERE ';

            foreach( $constraints as $field => $value ) {
                $SQL .= $field . ' = :' . $field . ' AND ';
            }

        }

        // Remove the final AND and prepare the statement
        $SQL = substr( $SQL, 0, -5 );       
        $PDOStatement = $this->PDO->prepare( $SQL );

        // Loop through constraints and bind parameters
        foreach( $constraints as $field => $value ) {
            print 'Binding ' . $field . ' to ' . $value . ' 
';
            $PDOStatement->bindParam( $field, $value );
        }

        $PDOStatement->execute();
        var_dump($PDOStatement);        

        while ( $results = $PDOStatement->fetch( PDO::FETCH_ASSOC ) ) {

            var_dump($results);

        }

    }

I'm pretty new to using PDO, but basically I'm attempting to pass an array of constraints e.g.

array( 'active' => 1, 'name' => 'James' )

and return all rows from the table

WHERE active = 1 AND name = 'James'

If I use this array, the SQL executed from the first

var_dump( )

is

SELECT * FROM {table} WHERE active = :active AND name = 'James'

- exactly as I expect. The bound parameters prints 'Binding active to 1' and 'Binding name to James' - exactly as expected. The rows exist in the database, and yet the second

var_dump()

call for $results outputs nothing - i.e. no rows are returned.

If I pass an array of a single constraint, e.g.

array( 'active' => 1 )

, this works perfectly fine. It appears to be whenever multiple constraints are passed that it stops working.

Answer 1

That's because bindParam works by binding to a variable, and you are re-using the variable ($value) for multiple values. Try with bindValue instead.

Or even better yet; Pass the values as an array to execute instead. This makes the statement stateless, which is generally a good thing in programming.

Answer 2

As mentioned, using bindValue instead of bindParam will certainly accomplish this. However, after spending a considerable amount of time troubleshooting this issue recently, I discovered an alternate solution. Here is how to accomplish PDO variable binding in a foreach loop using bindParam:

Replace the following line from the original post:

$PDOStatement->bindParam( $field, $value );

...with this:

$PDOStatement->bindParam( $field, $constraints[$field] );

Instead of binding $value, use $array_name[$array_key]. This works is because you are now binding to a unique variable instead of one that gets reused on each pass of the loop.

The variable $field used as the placeholder apparently does not need to be a unique variable, however. I have not thoroughly researched this yet, but a variable used as a placeholder appears to be parsed immediately (instead of being assigned as a variable reference) even when bindParam is used.

Also, as you would no longer need to access $value directly, you could also replace this:

foreach( $constraints as $field => $value ) {

... with this:

foreach (array_keys($constraints) as $field) {

This is optional, as it will work fine without this change. It looks cleaner in my opinion though, since it might get confusing later as to why $value is assigned but never used.