Checking for relative vs absolute paths/URLs in PHP

Question

I need to implement functions to check whether paths and urls are relative, absolute, or invalid (invalid syntactically- not whether resource exists). What are the range of

Answer 1

If you already know that the URL is well formed:

if(strpos($uri,'://')!==false){
    //protocol: absolute url
}elseif(substr($uri,0,1)!='/'){
    //leading '/': absolute to domain name (half relative)
}else{
    //no protocol and no leading slash: relative to this page
}