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PossiblyFalseOperand

Emitted when using a possibly false value as part of an operation (e.g. +, ., ^ etc).

<?php

function echoCommaPosition(string $str) : void {
    echo 'The comma is located at ' . strpos($str, ','); 
}

How to fix

You can detect the false value with some extra logic:

<?php

function echoCommaPosition(string $str) : void {
    $pos = strpos($str, ',');

    if ($pos === false) {
        echo 'There is no comma in the string';
    }

    echo 'The comma is located at ' . $pos; 
}

Alternatively you can just use a ternary to suppress this issue:

<?php

function echoCommaPosition(string $str) : void {
    echo 'The comma is located at ' . (strpos($str, ',') ?: ''); 
}