Make Behat recognise unnamed iframes on the page. We do not care how to name them, just how to operate with them.

FeatureContext.php

    /**
     * @Given I switch to iframe with locator :locator
     * @param String $locator
     */
    public function iSwitchToIFrameWithLocator($locator)
    {
        $value = $this->takeValueFromIniFile('locators', $locator);
        $iframe = $this->getSession()->getPage()->find("css", $value);
        $iframeName = $iframe->getAttribute("name");
        if ($iframeName == "") {
            echo "\n\niFrame has no name. Let's name it.\n\n";
            $javascript = "(function(){
            var iframes = document.getElementsByTagName('iframe');
            for (var i = 0; i < iframes.length; i++) {
                iframes[i].name = 'iframe_number_' + (i + 1) ;
            }
            })()";
            $this->getSession()->executeScript($javascript);
            $iframe = $this->getSession()->getPage()->find("css", $value);
            $iframeName = $iframe->getAttribute("name");
            echo "\n\niFrame has new name:  " . $iframeName . "\n\n";
        } else {
            echo "\n\niFrame already has a name: " . $iframeName . "\n\n";
        }
        
        $this->getSession()->getDriver()->switchToIFrame($iframeName);
    }