PHP: Move associative array element to beginning of array

You can use the array union operator (+) to join the original array to a new associative array using the known key (one).

$myArray = array('one' => $myArray['one']) + $myArray;
// or      ['one' => $myArray['one']] + $myArray;

Array keys are unique, so it would be impossible for it to exist in two locations.

See further at the doc on Array Operators:

The + operator returns the right-hand array appended to the left-hand array; for keys that exist in both arrays, the elements from the left-hand array will be used, and the matching elements from the right-hand array will be ignored.