EC
TypeError
PHPERRORCommonType

Argument or return type mismatch

Quick Answer

Enable declare(strict_types=1) and use typed properties to catch type mismatches at call sites.

What this means

Thrown in strict_types=1 mode when a function argument or return value does not match the declared type, or when a built-in function receives an argument of the wrong type.

Why it happens
  1. 1Passing a string where an int is declared
  2. 2Returning null from a non-nullable return type

Fix

Add strict_types and union types

Add strict_types and union types
<?php declare(strict_types=1);

function add(int $a, int $b): int {
    return $a + $b;
}

// Use int|string for flexible params
function format(int|string $val): string {
    return (string) $val;
}

Why this works

strict_types=1 makes type declarations strict; union types allow multiple accepted types.

Code examples
Triggerphp
<?php declare(strict_types=1);
function double(int $n): int { return $n * 2; }
double("5"); // TypeError
Nullable typephp
function find(?int $id): ?User {
    return $id ? User::find($id) : null;
}
Catch TypeErrorphp
try {
    setAge("old");
} catch (TypeError $e) {
    echo "Type error: " . $e->getMessage();
}
Same error in other languages
TS2345in TypeScript→TypeErrorin Python→

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