<?php
$origin = [2, 4, 2, 6];
foreach ($origin as $value) {
    $value = 1;
}
print_r($origin);

$origin = ['x', 4, 'x', 6];
foreach ($origin as $value) {
    $value = 1;
}
print_r($origin);

Array
(
    [0] => 2
    [1] => 4
    [2] => 2
    [3] => 6
)
Array
(
    [0] => x
    [1] => 4
    [2] => x
    [3] => 6
)

<?php
$origin = [2, 4, 2, 6];

$result = [];
foreach ($origin as $value) {

    // Cuando el valor es 2 se cambia por una 'x'
    if ($value == 2) {
        $result[] = 'x';
    } else {
        $result[] = $value;
    }
}
print_r($result);

<?php
$origin = [2, 4, 2, 6];

$result = [];
foreach ($origin as $value) {

    // Cuando el valor es 2 se cambia por una 'x'
    if ($value == 2) {
        $result[] = 'x';
        continue;
    }

    $result[] = $value;
}
print_r($result);

Array
(
    [0] => x
    [1] => 4
    [2] => x
    [3] => 6
)

<?php
$origin = [2, 4, 2, 6];

$result = [];
foreach ($origin as $value) {

    // Cuando el valor es 2 se cambia por una 'x'
    if ($value == 2) {
        $result[] = 'x';
        continue;
    }

    $result[] = $value;
}
print_r($result);

$origin = [];
foreach ($result as $value) {

    // Cuando el valor es 2 se cambia por una 'x'
    if ($value == 'x') {
        $origin[] = 2;
        continue;
    }

    $origin[] = $value;
}
print_r($origin);

Array
(
    [0] => x
    [1] => 4
    [2] => x
    [3] => 6
)
Array
(
    [0] => 2
    [1] => 4
    [2] => 2
    [3] => 6
)

<?php
$origin = [2, 4, 2, 6];

// Reasignar algunos valores pasandolos por referencia
foreach ($origin as &$value) {
    if ($value == 2)
        $value = 'x';
}
print_r($origin);

// Reasignar algunos valores pasandolos por referencia
foreach ($origin as &$value) {

    if ($value == 'x')
        $value = 1;
}
print_r($origin);

// Trato de reasignar todos los valores sin pasarlos por referencia
foreach ($origin as $value) {

    $value = 5;
}
print_r($origin);

Array
(
    [0] => x
    [1] => 4
    [2] => x
    [3] => 6
)
Array
(
    [0] => 1
    [1] => 4
    [2] => 1
    [3] => 6
)
Array
(
    [0] => 1
    [1] => 4
    [2] => 1
    [3] => 5
)