The easiest way: array_pop() which will pop an element of the end of the array.

As for the 2nd question:

if (end($transport) == "") { 
    array_pop($transport); 
}

Should handle the second.

EDIT:

Modified the code to conform to the updated information. This should work with associative or indexed based arrays.

Fixed the array_pop, given Scott's comment. Thanks for catching that!

Fixed the fatal error, I guess empty cannot be used with end like I had it. The above code will no longer catch null / false if that is needed you can assign a variable from the end function and test that like so:

$end_item = end($transport);
if (empty($end_item)) { 
    array_pop($transport); 
}

Sorry for posting incorrect code. The above I tested.

To remove the first element, use array_shift, to remove last element, use array_pop:

<?php    
$array = array('10', '20', '30.30', '40', '50');
array_shift($array);
array_pop($array);

Use explode instead of preg_split. It is faster. Then you can use array_pop and array_shift to remove an item from the end and beginning of the array. Then, use implode to put the remaining items back together again.

A better solution would be to use str_pos to find the first and last _ and use substr to copy the part inbetween. This will cause only one sting copy, instead of having to transform a string to array, modify that, and put the array together into a string. (Or don't you need to put them together? The 'I need 'os_disk' at the end confuses me).