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Strict mode and internal engine callbacks

General idea

The PHP engine is in 'weak' mode by default. This leads to a highly surprising behaviour when using internal functions that call a callback that was defined in a file that had strict mode enabled. For example:


declare(strict_types = 1);

$takesAnIntFn = function (string $x) {
    return $x;

$numbers = [1, 2, 3.6, 4];

var_dump(array_map($takesAnIntFn, $numbers));

echo "array_map worked\n";

$results = [];
foreach ($numbers as $number) {
    $results[] = $takesAnIntFn($number);

The output of that code is:

array(4) {
  string(1) "1"
  string(1) "2"
  string(3) "3.6"
  string(1) "4"
array_map worked

Fatal error: Uncaught TypeError: {closure}(): Argument #1 ($x) must be of type string, int given, called in /in/2A1e9 on line 18 and defined in /in/2A1e9:5
Stack trace:
#0 /in/2A1e9(18): {closure}(1)
#1 {main}
  thrown in /in/2A1e9 on line 5

What is happening is that when $takesAnIntFn is called directly from strict mode, the floats are correctly not cast to string automatically.

When $takesAnIntFn is called by the engine inside array_map, then it is called in weak mode, and the floats are cast to string automatically.

Failing to fail, is one of the worst ways that a programming language can fail, as it can lead to data corruption.

Hurdles to overcome

Someone needs to figure out what the correct behaviour is first, then figure out how to get to that correct behaviour without too huge a BC break.


This needs to be fixed some day. It might be really quite tricky to do.


A thread on how surprising it is: