In PHP 5.3.0, `is_a()` is no longer deprecated, and will therefore no longer throw `E_STRICT` warnings.
To avoid warnings in PHP < 5.3.0, convert all `is_a()` calls to `$var instanceof WP_Class` calls.
`instanceof` does not throw any error if the variable being tested is not an object, it simply returns `false`.
Props markoheijnen, wonderboymusic.
Fixes#25672.
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