function makeClickableLinks($text) {  
 $text = eregi_replace('(((f|ht){1}tp://)[-a-zA-Z0-9@:%_+.~#?&//=]+)',  
 '<a href="\1">\1</a>', $text);  
 $text = eregi_replace('([[:space:]()[{}])(www.[-a-zA-Z0-9@:%_+.~#?&//=]+)',  
 '\1<a href="http://\2">\2</a>', $text);  
 $text = eregi_replace('([_.0-9a-z-]+@([0-9a-z][0-9a-z-]+.)+[a-z]{2,3})',  
 '<a href="mailto:\1">\1</a>', $text);  
  
return $text;  
}

Convert URLs within String into hyperlinks

PHP