You can use this if you don't care about keeping the correct array keys

function flattenA(array $array) {
    $return = array();
    array_walk_recursive($array, function($a) use (&$return) { $return[] = $a; });
    return $return;
}
print_r(flattenA($arr));
// Output
Array
(
    [0] => test1
    [1] => test2
    [2] => test3
    [3] => test4
    [4] => test5
)

Otherwise

function flattenB(array $array) {
    $return = array();
    array_walk_recursive($array, function($v,$k) use (&$return) { $return[$k] = $v; });
    return $return;
}
print_r(flattenB($arr));
// Output
Array
(
    [t1] => test1
    [t2] => test2
    [t3] => test3
    [t4] => test4
    [t5] => test5
)

Check both on Sandbox

From answer on similar question

You can use this if you don't care about keeping the correct array keys

function flattenA(array $array) {
    $return = array();
    array_walk_recursive($array, function($a) use (&$return) { $return[] = $a; });
    return $return;
}
print_r(flattenA($arr));
// Output
Array
(
    [0] => test1
    [1] => test2
    [2] => test3
    [3] => test4
    [4] => test5
)

Otherwise

function flattenB(array $array) {
    $return = array();
    array_walk_recursive($array, function($v,$k) use (&$return) { $return[$k] = $v; });
    return $return;
}
print_r(flattenB($arr));
// Output
Array
(
    [t1] => test1
    [t2] => test2
    [t3] => test3
    [t4] => test4
    [t5] => test5
)

Check both on Sandbox

From answer on similar question

function flatten(array $array) {
    $return = array();
    array_walk_recursive($array, function($a) use (&$return) { $return[] = $a; });
    return $return;
}

From answer on similar question

You can use this if you don't care about keeping the correct array keys

function flattenA(array $array) {
    $return = array();
    array_walk_recursive($array, function($a) use (&$return) { $return[] = $a; });
    return $return;
}
print_r(flattenA($arr));
// Output
Array
(
    [0] => test1
    [1] => test2
    [2] => test3
    [3] => test4
    [4] => test5
)

Otherwise

function flattenB(array $array) {
    $return = array();
    array_walk_recursive($array, function($v,$k) use (&$return) { $return[$k] = $v; });
    return $return;
}
print_r(flattenB($arr));
// Output
Array
(
    [t1] => test1
    [t2] => test2
    [t3] => test3
    [t4] => test4
    [t5] => test5
)

Check both on Sandbox

From answer on similar question