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    <p>You can use <a href="http://www.php.net/manual/en/language.operators.type.php"><code>instanceof</code></a>:</p> <pre><code>if ($pdo instanceof PDO) { // it's PDO } </code></pre> <p>Be aware though, you can't negate like <code>!instanceof</code>, so you'd instead do:</p> <pre><code>if (!($pdo instanceof PDO)) { // it's not PDO } </code></pre> <p>Also, looking over your question, you can use object type-hinting, which helps enforce requirements, as well as simplify your check logic:</p> <pre><code>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 </code></pre> <p>Typed arguments can be required or optional:</p> <pre><code>// 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 $pdo = null) { } </code></pre> <p>Untyped arguments allow for flexibility through explicit conditions:</p> <pre><code>// accepts any argument, checks for PDO in body function connect($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')) { // ... } } </code></pre> <p>As for the latter (<em>using <a href="http://php.net/manual/en/function.method-exists.php"><code>method_exists</code></a></em>), I'm a bit mixed in my opinion. People coming from Ruby would find it familiar to <a href="http://ruby-doc.org/core-2.1.1/Object.html#method-i-respond_to-3F"><code>respond_to?</code></a>, for better or for worse. I'd personally write an interface and perform a normal type-hint against that:</p> <pre><code>interface QueryableInterface { function query(); } class MyPDO extends PDO implements QueryableInterface { } function connect(QueryableInterface $queryable) { } </code></pre> <p>However, that's not <em>always</em> feasible; in this example, <code>PDO</code> objects are not valid parameters as the base type doesn't implement <code>QueryableInterface</code>.</p> <p>It's also worth mentioning that <em>values have types</em>, not variables, in PHP. This is important because <code>null</code> will fail an <code>instanceof</code> check.</p> <pre><code>$object = new Object(); $object = null; if ($object instanceof Object) { // never run because $object is simply null } </code></pre> <p>The value loses it's type when it becomes <code>null</code>, a lack of type.</p>
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    1. USDan Lugg
    1. USDan Lugg
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    1. COI thought type-hinting was not allowed in PHP
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    2. COIt sure is, but only for `Array` and object class types. So `PDO` and any user-land classes are good, as in my example. You can't hint scalars though, such as `string`.
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    3. COI see, though I don't want a fatal error, I want an exception thrown. Good to know though :)
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