Which PHP Construct Is Better for Verifying Array Element Existence: isset or array_key_exists?

Using isset or array_key_exists to Verify Array Element Existence

When seeking to ascertain the presence of an array element, you can utilize either the isset language construct or the array_key_exists function.

isset

This approach is potentially more efficient because it is not a function. However, it may return false if the element exists but has a value of NULL.

Consider the following array:

<code class="php">$a = array(
    123 => 'glop', 
    456 => null, 
);</code>

Using isset to test for element existence:

<code class="php">var_dump(isset($a[123])); // true
var_dump(isset($a[456])); // false
var_dump(isset($a[789])); // false</code>

array_key_exists

In contrast, array_key_exists evaluates solely the presence of the key, regardless of its value.

Using array_key_exists with the same array:

<code class="php">var_dump(array_key_exists(123, $a)); // true
var_dump(array_key_exists(456, $a)); // true
var_dump(array_key_exists(789, $a)); // false</code>

Choosing the Right Approach

For scenarios where the element is guaranteed to have a non-NULL value, isset may be preferable due to its efficiency. Otherwise, array_key_exists may be more suitable.

In your specific example, you can modify your code as follows using isset:

<code class="php">if (!isset(self::$instances[$instanceKey])) {
    $instances[$instanceKey] = $theInstance;
}</code>

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