In PHP, how can one determine if a specific key exists in an array before outputting its value?

To determine if a specific key exists in an array before outputting its value, you can use the `array_key_exists()` function in PHP. This function checks if a specific key exists in the given array and returns a boolean value (true if the key exists, false if it does not). By using this function, you can avoid errors or warnings when trying to access a key that does not exist in the array.

// Example array
$array = array(
    'key1' => 'value1',
    'key2' => 'value2'
);

// Check if 'key1' exists in the array before outputting its value
if (array_key_exists('key1', $array)) {
    echo $array['key1'];
} else {
    echo 'Key does not exist';
}