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MixedOperand

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

<?php

echo $GLOBALS['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