How to Reliably Check for Variable Existence in PHP?

Reliable Variable Existence Checking in PHP

The isset() function, while commonly used to verify variable existence, has a limitation: it considers variables set to NULL as existing. This raises the question of how to reliably determine a variable's presence in PHP.

One approach is to combine isset() with is_null():

<code class="php">if (isset($v) || @is_null($v))</code>

However, this method remains problematic due to is_null()'s behavior with unset variables.

Another option is to use the @($v === NULL) comparison. However, this also behaves like is_null().

For a more reliable approach, consider using array_key_exists(). This function operates correctly for both global variables and arrays:

<code class="php">$a = NULL;
var_dump(array_key_exists('a', $GLOBALS)); // true
var_dump(array_key_exists('b', $GLOBALS)); // false</code>

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