What is causing the "Undefined array key" PHP warning in the provided code snippet?

The "Undefined array key" PHP warning is caused when trying to access an array element using a key that does not exist in the array. To solve this issue, you should first check if the key exists in the array before accessing it to avoid the warning. This can be done using the isset() function to verify the existence of the key in the array. 

// Check if the key exists before accessing it to avoid "Undefined array key" warning
if(isset($array['key'])) {
    // Access the array element if the key exists
    $value = $array['key'];
    // Use the $value variable as needed
} else {
    // Handle the case when the key does not exist in the array
    echo "Key does not exist in the array";
}

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PHP Undefined array key warning code snippet debugging

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