JuniorTheoryCommon

What happens when you run this program?

PHP

<?php

declare(strict_types=1);

function half(float $x): float
{
    return $x / 2;
}

echo half(5), ' ', half('5'), "\n";

2.5, then a TypeError for '5'

Expecting '5' to pass because it holds a number: under strict_types a string is never converted, so half('5') throws a TypeError.
Expecting half(5) to fail because 5 is an int: the one widening strict mode allows is from int to float, so 5 arrives as 5.0.

Which file decides whether a call is checked strictly: the caller's or the function's?
Why must declare(strict_types=1) be the first statement of the file?

JuniorTheoryCommon

PHP

<?php

declare(strict_types=1);

function half(float $x): float
{
    return $x / 2;
}

echo half(5), ' ', half('5'), "\n";

2.5, then a TypeError for '5'

Expecting '5' to pass because it holds a number: under strict_types a string is never converted, so half('5') throws a TypeError.
Expecting half(5) to fail because 5 is an int: the one widening strict mode allows is from int to float, so 5 arrives as 5.0.

Which file decides whether a call is checked strictly: the caller's or the function's?
Why must declare(strict_types=1) be the first statement of the file?