How to Check for a Specific Type of Object in PHP

Question

I have a method which accepts a PDO object as an argument, to allow the user to use an existing connection rather then the method to open a new one, and save resources:

public static function databaseConnect($pdo = null) {

I am aware of is_object() to check if the argument is an object, but I want to check if $pdo is a PDO object, and not just an object.

Because the user can easily enter (by mistake?) a different kind of object, a mysqli or such, and the entire script will break apart.

In short: How can I check a variable for a specific type of object?

Answer 1

Hello @kartik,

You can use instanceof:

if ($pdo instanceof PDO) {
    // it's PDO
}

Be aware though, you can't negate like !instanceof, so you'd instead do:

if (!($pdo instanceof PDO)) {
    // it's not PDO
}

Also, looking over your question, you can use object type-hinting, which helps enforce requirements, as well as simplify your check logic:

function connect(PDO $pdo = null)
{
    if (null !== $pdo) {
        // it's PDO since it can only be
        // NULL or a PDO object (or a sub-type of PDO)
    }
}

connect(new SomeClass()); // fatal error, if SomeClass doesn't extend PDO

Typed arguments can be required or optional:

// required, only PDO (and sub-types) are valid
function connect(PDO $pdo) { }

// optional, only PDO (and sub-types) and 
// NULL (can be omitted) are valid
function connect(PDO $pdo = null) { }

Untyped arguments allow for flexibility through explicit conditions:

// accepts any argument, checks for PDO in body
function connect($pdo)
{
    if ($pdo instanceof PDO) {
        // ...
    }
}

// accepts any argument, checks for non-PDO in body
function connect($pdo)
{
    if (!($pdo instanceof PDO)) {
        // ...
    }
}

// accepts any argument, checks for method existance
function connect($pdo)
{
    if (method_exists($pdo, 'query')) {
        // ...
    }
}

Hope it helps!!