psalm/docs/running_psalm/issues/UnusedVariable.md

UnusedVariable

Emitted when --find-dead-code is turned on and Psalm cannot find any references to a variable, once instantiated

<?php

function foo() : void {
    $a = 5;
    $b = 4;
    echo $b;
}

Can be suppressed by prefixing the variable name with an underscore:

<?php

$_a = 22;