ParseURL.php

<?php
//Write your PHP code here
$url = "http://stackoverflow.com/questions/6240414/add-http-prefix-to-url-when-missing";

$scheme = parse_url($url, PHP_URL_SCHEME);
if (empty($scheme)) {
    $url = 'http://' . ltrim($url, '/');
}

$urlnohttp = preg_replace("(^https?://)", "", $url );

echo "URL: ". $url; // URL: http://stackoverflow.com/questions/6240414/add-http-prefix-to-url-when-missing
echo "<br>"; 
echo "scheme: ". $scheme; // scheme: http
echo "<br>"; 
echo "urlnohttp: ". $urlnohttp; // urlnohttp: stackoverflow.com/questions/6240414/add-http-prefix-to-url-when-missing
echo "<br>"; 

// FOR MY NEED
if ($scheme) {
    echo "Get http or https<br>"; // Get http or https
}

//$parse = parse_url($url);
//print "parse: ". $parse['host']; // prints 'stackoverflow.com'
?>