Move an array element to a new index in PHP

As commented, 2x array_splice, there even is no need to renumber:

$array = [
    0 => 'a', 
    1 => 'c', 
    2 => 'd', 
    3 => 'b', 
    4 => 'e',
];

function moveElement(&$array, $a, $b) {
    $out = array_splice($array, $a, 1);
    array_splice($array, $b, 0, $out);
}

moveElement($array, 3, 1);

redzarf comments: “To clarify $a is $fromIndex and $b is $toIndex“

Result:

[
    0 => 'a',
    1 => 'b',
    2 => 'c',
    3 => 'd',
    4 => 'e',
];