<?php
declare(strict_types=1);

class Test
{
    public function error(?string $someString): void
    {
        // Line below is the only difference to "pass" method
        if (!$someString) {
            return;
        }

        // Strict comparison using === between int<min, -1>|int<1, max> and 0 will always evaluate to false.
        $firstLetterAsInt = (int)substr($someString, 0, 1);
        if ($firstLetterAsInt === 0) {
            return;
        }
    }

    public function pass(?string $someString): void
    {
        // Line below is the only difference to "error" method
        if ($someString === null) {
            return;
        }

        // All ok
        $firstLetterAsInt = (int)substr($someString, 0, 1);
        if ($firstLetterAsInt === 0) {
            return;
        }
    }
}