En general, eso se hace con expresiones regulares.
Un ejemplo de cómo lo podés hacer
Código PHP:
<?php
/**
NAME : autolink()
VERSION : 1.0
AUTHOR : J de Silva
DESCRIPTION : returns VOID; handles converting
URLs into clickable links off a string.
TYPE : functions
======================================*/
function autolink( &$text, $target='_blank', $nofollow=true )
{
// grab anything that looks like a URL...
$urls = _autolink_find_URLS( $text );
if( !empty($urls) ) // i.e. there were some URLS found in the text
{
array_walk( $urls, '_autolink_create_html_tags', array('target'=>$target, 'nofollow'=>$nofollow) );
$text = strtr( $text, $urls );
}
}
function _autolink_find_URLS( $text )
{
// build the patterns
$scheme = '(http:\/\/|https:\/\/)';
$www = 'www\.';
$ip = '\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}';
$subdomain = '[-a-z0-9_]+\.';
$name = '[a-z][-a-z0-9]+\.';
$tld = '[a-z]+(\.[a-z]{2,2})?';
$the_rest = '\/?[a-z0-9._\/~#&=;%+?-]+[a-z0-9\/#=?]{1,1}';
$pattern = "$scheme?(?(1)($ip|($subdomain)?$name$tld)|($www$name$tld))$the_rest";
$pattern = '/'.$pattern.'/is';
$c = preg_match_all( $pattern, $text, $m );
unset( $text, $scheme, $www, $ip, $subdomain, $name, $tld, $the_rest, $pattern );
if( $c )
{
return( array_flip($m[0]) );
}
return( array() );
}
function _autolink_create_html_tags( &$value, $key, $other=null )
{
$target = $nofollow = null;
if( is_array($other) )
{
$target = ( $other['target'] ? " target=\"$other[target]\"" : null );
// see: http://www.google.com/googleblog/2005/01/preventing-comment-spam.html
$nofollow = ( $other['nofollow'] ? ' rel="nofollow"' : null );
}
$value = "<a href=\"$key\"$target$nofollow>$key</a>";
}
?>