Analyse -> Cannot resolve file 'EXTERNAL_FRAGMENT'

For this code:

<?php
foreach($Bandeaux as $Bandeau) {
    $url = "/img/".$Bandeau['Lien_Image'];
        echo "<div style='background-image: url($url);'></div>";
}
?>

Run "Analyse" -> "Inspect code" return "Cannot resolve file 'EXTERNAL_FRAGMENT'" for $url.

I have no idea what i can do for revise this code.