PHP: image real path

<?php
echo "<h1>Hello, World</h1>" . PHP_EOL;
echo "<p>Welcome</p>" . PHP_EOL;

$img = "{$_SERVER["REQUEST_SCHEME"]}://{$_SERVER["HTTP_HOST"]}/uploads/qtalk.jpg";
$prefix = $_SERVER["REQUEST_SCHEME"] . "://" . $_SERVER["HTTP_HOST"] . "/";
echo "$prefix<br>" . PHP_EOL;
if(0 === strpos($img, $prefix)){
    $img = substr($img, strlen($prefix));
    $path = realpath($img); 
    echo "img = $img\tpath = $path<br>" . PHP_EOL;
}

$img = "//{$_SERVER["HTTP_HOST"]}/uploads/qtalk.jpg";
$prefix1 = "//" . $_SERVER["HTTP_HOST"] . "/";
if(0 === strpos($img, $prefix1)){
    $img = substr($img, strlen($prefix1));
    $path = realpath($img); 
    echo "img = $img\tpath = $path<br>" . PHP_EOL;
}

$img = "/uploads/qtalk.jpg";
$prefix2 = "/";
if(0 === strpos($img, $prefix2)){
    $img = substr($img, strlen($prefix2));
    $path = realpath($img); 
    echo "img = $img\tpath = $path<br>" . PHP_EOL;
}