How to extract complete sub links using Simple-HTM

Question

The following is the basic code I use to extract sublinks from a page:

<?php
    include_once('simple_html_dom.php');
    function extract_links($target_url)
    {   
        $html = new simple_html_dom();
        $html->load_file($target_url);  
        $i=0;
        $crawl =array();
        foreach($html->find('a') as $link)
        {
            $crawl[$i] = $link->href;
            $i++;
        }
        var_dump($crawl);
    }
    extract_links('http://stackoverflow.com');
?>

The output is as follows:

array
  0 => string 'http://stackexchange.com' (length=24)
  1 => string '/users/login' (length=12)
  2 => string 'http://careers.stackoverflow.com' (length=32)
  3 => string 'http://chat.stackoverflow.com' (length=29)
  4 => string 'http://meta.stackoverflow.com' (length=29)
  5 => string '/about' (length=6)
  6 => string '/faq' (length=4)
  7 => string '/' (length=1)
  8 => string '/questions' (length=10)
  9 => string '/tags' (length=5)
  10 => string '/users' (length=6)
  11 => string '/badges' (length=7)
  12 => string '/unanswered' (length=11)
  13 => string '/questions/ask' (length=14)
  14 => string '?tab=interesting' (length=16)
  15 => string '?tab=featured' (length=13)
  16 => string '?tab=hot' (length=8)
  17 => string '?tab=week' (length=9)
  18 => string '?tab=month' (length=10)
  19 => string '/questions/14611052/basic-standalone-jpa-example-with-postgres-using-eclipse' (length=76)
  20 => string '/questions/tagged/eclipse' (length=25)
  21 => string '/questions/tagged/postgresql' (length=28)
  22 => string '/questions/tagged/jpa' (length=21)
  23 => string '/questions/14611052/basic-standalone-jpa-example-with-postgres-using-eclipse' (length=76)
  24 => string '/users/865448/tostao' (length=20)
  25 => string '/questions/14611172/unable-to-fully-print-a-page-containing-iframes-in-chrome' (length=77)
  26 => string '/questions/tagged/javascript' (length=28)
  27 => string '/questions/tagged/jquery' (length=24)
  28 => string '/questions/tagged/html' (length=22)
  29 => string '/questions/tagged/html5' (length=23)
  30 => string '/questions/tagged/google-chrome' (length=31)
  31 => string '/questions/14611172/unable-to-fully-print-a-page-containing-iframes-in-chrome' (length=77)
  32 => string '/users/962868/tejas' (length=19)
  33 => string '/questions/14609779/how-can-i-configure-bash-to-handle-crlf-shell-scripts' (length=73)
  34 => string '/questions/tagged/linux' (length=23)
  35 => string '/questions/tagged/windows' (length=25)
  36 => string '/questions/tagged/bash' (length=22)
  37 => string '/questions/tagged/line-endings' (length=30)
  38 => string '/questions/14609779/how-can-i-configure-bash-to-handle-crlf-shell-scripts/?lastactivity' (length=87)
  39 => string '/users/1899640/that-other-guy' (length=29)
  40 => string '/questions/14611169/using-one-socket-for-peer-to-peer-communication' (length=67)
  41 => string '/questions/tagged/sockets' (length=25)
  42 => string '/questions/tagged/p2p' (length=21)
  43 => string '/questions/14611169/using-one-socket-for-peer-to-peer-communication' (length=67)
  44 => string '/users/911651/xsnrg' (length=19)
  45 => string '/questions/14611166/possible-mistake-in-ios-dev-guide' (length=53)
  46 => string '/questions/tagged/iphone' (length=24)
  47 => string '/questions/tagged/ios' (length=21)
  48 => string '/questions/tagged/objective-c' (length=29)
  49 => string '/questions/14611166/possible-mistake-in-ios-dev-guide' (length=53)
  50 => string '/users/107715/matt-n' (length=20)
  51 => string '/questions/14611163/how-to-use-dispatcher-in-wpf-to-make-a-timer' (length=64)
  52 => string '/questions/tagged/wpf' (length=21)
  53 => string '/questions/tagged/timer' (length=23)
  54 => string '/questions/tagged/dispatcher' (length=28)
  55 => string '/questions/14611163/how-to-use-dispatcher-in-wpf-to-make-a-timer' (length=64)
  56 => string '/users/1741800/nashat' (length=21)
  57 => string '/questions/14610879/how-can-i-handle-an-access-violation-in-visual-studio-c' (length=75)
  58 => string '/questions/tagged/visual-c%2b%2b' (length=32)
  59 => string '/questions/tagged/exception-handling' (length=36)
  60 => string '/questions/tagged/access-violation' (length=34)
  61 => string '/questions/tagged/structured-exception' (length=38)
  62 => string '/questions/14610879/how-can-i-handle-an-access-violation-in-visual-studio-c/?lastactivity' (length=89)
  63 => string '/users/901812/big-endian' (length=24)
  64 => string '/questions/14611162/mvc-condintional-authorization' (length=50)
  65 => string '/questions/tagged/c%23' (length=22)
  66 => string '/questions/tagged/asp.net-mvc' (length=29)
  67 => string '/questions/tagged/asp.net-mvc-4' (length=31)
  68 => string '/questions/tagged/authorization' (length=31)
  69 => string '/questions/14611162/mvc-condintional-authorization' (length=50)
  70 => string '/users/644969/cadrell0' (length=22)
  71 => string '/questions/14611160/get-customer-role-nopcommerce' (length=49)
  72 => string '/questions/tagged/c%23' (length=22)
  73 => string '/questions/tagged/razor' (length=23)
  74 => string '/questions/tagged/nopcommerce' (length=29)
  75 => string '/questions/14611160/get-customer-role-nopcommerce' (length=49)
  76 => string '/users/1378841/mlg74' (length=20)
  77 => string '/questions/14611158/iframe-resizing-nested-in-gridview' (length=54)
  78 => string '/questions/tagged/resize' (length=24)
  79 => string '/questions/14611158/iframe-resizing-nested-in-gridview' (length=54)
  80 => string '/users/2026451/satish-patil' (length=27)
  81 => string '/questions/14611157/php-how-to-check-the-value-got-this-word-from-a-var' (length=71)
  82 => string '/questions/tagged/php' (length=21)
  83 => string '/questions/tagged/preg-match' (length=28)
  84 => string '/questions/tagged/strpos' (length=24)
  85 => string '/questions/14611157/php-how-to-check-the-value-got-this-word-from-a-var' (length=71)
  86 => string '/users/963414/samual99' (length=22)
  87 => string '/questions/14611155/how-to-get-the-coordinates-of-boundries-of-drawable-on-the-mapview' (length=86)
  88 => string '/questions/tagged/android' (length=25)
  89 => string '/questions/tagged/google-maps' (length=29)
  90 => string '/questions/14611155/how-to-get-the-coordinates-of-boundries-of-drawable-on-the-mapview' (length=86)
  91 => string '/users/1520564/blubar' (length=21)
  92 => string '/questions/14611153/why-css-is-empty-when-ssl-is-on-and-appcache-is-enabled-ipad-safari' (length=87)
  93 => string '/questions/tagged/css' (length=21)
  94 => string '/questions/tagged/ipad' (length=22)
  95 => string '/questions/tagged/ssl' (length=21)
  96 => string '/questions/tagged/mobile-safari' (length=31)
  97 => string '/questions/tagged/html5-appcache' (length=32)
  98 => string '/questions/14611153/why-css-is-empty-when-ssl-is-on-and-appcache-is-enabled-ipad-safari' (length=87)
  99 => string '/users/2026375/twoface' (length=22)
  100 => string '/questions/14611149/laravel-how-to-temporarily-store-eloquent-models-in-db-without-a-proper-schem' (length=97)
  101 => string '/questions/tagged/php' (length=21)
  102 => string '/questions/tagged/laravel' (length=25)
  103 => string '/questions/14611149/laravel-how-to-temporarily-store-eloquent-models-in-db-without-a-proper-schem' (length=97)
  104 => string '/users/291557/duality' (length=21)
  105 => string '/questions/13928812/xmlserializer-generateserializer-and-collections' (length=68)
  106 => string '/questions/tagged/c%23' (length=22)
  107 => string '/questions/tagged/xml-serialization' (length=35)
  108 => string '/questions/13928812/xmlserializer-generateserializer-and-collections/?lastactivity' (length=82)
  109 => string '/users/1200614/phil' (length=19)
  110 => string '/questions/14611145/keep-buttons-in-view-when-keyboard-opens-android' (length=68)
  111 => string '/questions/tagged/android' (length=25)
  112 => string '/questions/tagged/keyboard' (length=26)
  113 => string '/questions/tagged/resize' (length=24)
  114 => string '/questions/tagged/window' (length=24)
  115 => string '/questions/tagged/views' (length=23)
  116 => string '/questions/14611145/keep-buttons-in-view-when-keyboard-opens-android' (length=68)
  117 => string '/users/1137413/725623452362' (length=27)
  118 => string '/questions/14611144/ssdp-discovery-from-a-browser' (length=49)
  119 => string '/questions/tagged/silverlight' (length=29)
  120 => string '/questions/tagged/flash' (length=23)
  121 => string '/questions/14611144/ssdp-discovery-from-a-browser' (length=49)
  122 => string '/users/191882/legege' (length=20)
  123 => string '/questions/14611143/how-to-syncrhonize-on-site-in-memory-no-sql-datasources-with-central-database-in' (length=100)
  124 => string '/questions/tagged/architecture' (length=30)
  125 => string '/questions/tagged/nosql' (length=23)
  126 => string '/questions/tagged/java-ee-6' (length=27)
  127 => string '/questions/tagged/in-memory-database' (length=36)
  more elements...

Now consider '/about' sublink in the array. I want it to be displayed as 'https://stackoverflow.com/about'. Why only subpart of sublink is returned while in some cases complete sublink is returned ? Also some links are starting with '?' sign. How to sanitize these links ?

EDIT: Consider "http://en.wikipedia.org/wiki/Web_crawler". Now if I perform extract_links on it, I get a sublink like this "http://en.wikipedia.org/wiki/Web_crawler/wiki/Web_search_engine" which is invalid and most of the links are of this format. The correct link is "http://en.wikipedia.org/wiki/Web_search_engine". And I am using this function in another program which will pass an array of links so I cannot keep the if conditions static. The following is the code fragment I am using now:

foreach($html->find('a') as $link)
{   
    $href = $link->href;
    $fchr = substr($href, 0, 1);
    if ($fchr === '/')
    {
        $href = $target_url.$href;
    }
    else if ($fchr === '?')
    {
        $href = $target_url.'/'. $href;
    }
}

Answer 1

In html source, definition of this a element is <a href="/about">about</a>, thus it returned existing href value as /about.

If you want to display links with http and domain try this (all internal links could start with / or ?);

foreach($html->find('a') as $link) {
    $href = $link->href;
    $fchr = substr($href, 0, 1);
    if ($fchr === '/') {
        $href = 'http://stackoverflow.com'. $href;
    } else if ($fchr === '?') {
        $href = 'http://stackoverflow.com/'. $href;
    }
    $crawl[] = $href;
}

PN: You don't need $i while pushing non-assoc arrays, just $array[] = .. is ok.

UPDATE

Assuming you have trouble with "internal links" (so, these are may not contain http or site url). And actually I don't know how to detect internal links anymore. I think this grabs all those: 'foo', '/foo', '?foo', '#foo', '../foo', '/../foo'. But suppose these seems not valid?? '../foo', '/../foo'.

// or more strong pattern
if (!preg_match('~^((ht|f)tp?s*)://.*~i', trim($href))) {
    $href = 'http://www.site.com/'. ltrim($href, '/');
}

Answer 2

@pguardiario's comment

As suggested in his comment phpUri is the perfect solution for converting relative URLs to absolute. You can find it here,

Answer 3

Any link starting with "/" is an absolute path from the doc root. To get the complete URL you would need to prepend the hostname in which that link was found. For relative links, such as the "?tab=etc", you will need to prepend the complete URL in which the link was found. If you want to ignore the query string links ("?tab=etc") use a regular expression to do so.