PHP Arrays: A good way to check if an array is associative or sequential?

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 = array('apple', 'orange', 'tomato', 'carrot');
 

and this:

$assocArray = array('fruit1' => 'apple', 
                     'fruit2' => 'orange', 
                     'veg1' => 'tomato', 
                     'veg2' => 'carrot');
 

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