check if a string is a URL

PHP's filter_var function is what you need. Look for FILTER_VALIDATE_URL. You can also set flags to fine-tune your implementation.
No regex needed....

The code below worked for me:

if(filter_var($text, FILTER_VALIDATE_URL))
{
    echo "Yes it is url";
    exit; // die well
}
else
{
    echo "No it is not url";
   // my else codes goes
}

You can also specify RFC compliance and other requirements on the URL using flags. See PHP Validate Filters for more details.