psalm/docs/running_psalm/issues/MixedOperand.md

MixedOperand

Emitted when Psalm cannot infer a type for an operand in any calculated expression

<?php

echo $_GET['foo'] + "hello";

Why it’s bad

Mixed operands can have fatal consequences, e.g. here:

<?php

function foo(mixed $m) {
    echo $m . 'bar';
}

class A {}

foo(new A()); // triggers fatal error