PHP treats all arrays as associative, so there aren't any built in functions. Can anyone recommend a fairly efficient way to check if an array contains only numeric keys?

Basically, I want to be able to differentiate between this:

$sequentialArray = [
    'apple', 'orange', 'tomato', 'carrot'
];

and this:

$assocArray = [
    'fruit1' => 'apple',
    'fruit2' => 'orange',
    'veg1' => 'tomato',
    'veg2' => 'carrot'
];