What is the potential cause of the 'Invalid argument supplied for foreach()' warning in PHP?

The 'Invalid argument supplied for foreach()' warning in PHP typically occurs when trying to loop through a variable that is not an array or an object that does not implement the Traversable interface. To solve this issue, you should check if the variable is an array or object before using foreach to iterate over it.

if(is_array($variable) || $variable instanceof Traversable) {
    foreach($variable as $item) {
        // code to process each item
    }
} else {
    // handle the case where $variable is not iterable
}