A foreach() always assigns the value variable before assigning the key variable. (Demo)

This seems a little counterintuitive because $v visually appears to be declared after it is access by key declaration.

This means you can safely use:

$array = ['a', 'b'];
$result = [];
foreach ($array as $result["prefix_$v"] => $v);
var_export($result);

to produce:

array (
  'prefix_a' => 0,
  'prefix_b' => 1,
)

The newly generated array has prefixed keys based on the original values and the original indexes become the new values.

Without the prefix_ prepended to the string, the action is effectively the same as array_flip().

Reversing the assignments so that the value variable receives the key does not work as intended. You might hope to generate ["prefix_0", "prefix_1"] but it does not and it also generates Notices/Warnings due to trying to access an undeclared variable. (The complaint is only on the first iteration because after the first iteration, $k is declared -- it is the previous iteration's key.)

$array = ['a', 'b'];
$result = [];
foreach ($array as $k => $result["prefix_$k"]);
// not defined on first iteration -------^^
var_export($result);

Bad Output:

Warning: Undefined variable $k in /in/iVm1u on line 6
array (
  'prefix_' => 'a',
  'prefix_0' => 'b',
)

For additional context, I'll offer one more working example that generates a multi-dimensional array using the value to determine the array keys in the output array. (Demo)

$array = [2, 3, 5];

$result = [];
foreach ($array as $result[$v * $v][$v] => $v);
var_export($result);

Output:

array (
  4 => 
  array (
    2 => 0,
  ),
  9 => 
  array (
    3 => 1,
  ),
  25 => 
  array (
    5 => 2,
  ),
)