When declaring an Array in PHP, the index's may be created out of order...I.e

Array[1] = 1 Array[19] = 2 Array[4] = 3

My question. In creating an array like this, is the length 19 with nulls in between? If I attempted to get Array[3] would it come as undefined or throw an error? Also, how does this affect memory. Would the memory of 3 index's be taken up or 19?

Also currently a developer wrote a script with 3 arrays FailedUpdates[] FailedDeletes[] FailedInserts[]

Is it more efficient to do it this way, or do it in the case of an associative array controlling several sub arrays

"Failures" array(){
    ["Updates"] => array(){
           [0] => 12
           [1] => 41
                   }
    ["Deletes"] => array(){
           [0] => 122
           [1] => 414
           [1] => 43
                   }
    ["Inserts"] => array(){
           [0] => 12
                   }
}