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declare(strict_types=1);

<?php
// Coercive mode
function sum(int ...$ints)
{
   return array_sum($ints);
}

print(sum(2, '3', 4.1));
?>

9

<?php
// Strict mode
declare(strict_types=1);

function sum(int ...$ints)
{
   return array_sum($ints);
}

print(sum(2, '3', 4.1));
?>

PHP Fatal error:  Uncaught TypeError: Argument 2 passed to sum() must be of the type integer, string given, called in……

<?php
declare(strict_types=1);

function returnIntValue(int $value): int
{
   return $value;
}

print(returnIntValue(5));
?>

5

<?php
declare(strict_types=1);

function returnIntValue(int $value): int
{
   return $value + 1.0;
}

print(returnIntValue(5));
?>

Fatal error: Uncaught TypeError: Return value of returnIntValue() must be of the type integer, float returned...

<?php
function swap(&$left, &$right) : void
{
    if ($left === $right) {
        return;
    }

    $tmp = $left;
    $left = $right;
    $right = $tmp;
}

$a = 1;
$b = 2;
var_dump(swap($a, $b), $a, $b);

null
int(2)
int(1)