I think all understand :)

Example, type: //echo $url_encode(тест)
in mIRC >= v7.0

Returns: %D1%82 %D0%B5 %D1%81 %D1%82 (without spaces, of course)
(two bytes for one unicode russian symbol)

alias url_encode {
  var %i = 1, %len = $len($1), %str
  while (%i <= %len) {
    var %char = $asc($mid($1,%i,1)), %i = %i + 1
    if (%char isnum 33-47) || (%char isnum 48-57) || (%char isnum 58-64) || (%char isnum 65-90) || (%char isnum 97-122) var %str = $+(%str,$chr(%char))
    elseif (%char < 128) var %str = $+(%str,%,$base(%char,10,16,2))
    elseif (%char < 2048) var %str = $+(%str,%,$base($calc(192 + $int($calc(%char / 64))),10,16,2),%,$base($calc(128 + (%char % 64)),10,16,2))
    elseif (%char < 65536) var %str = $+(%str,%,$base($calc(224 + $int($calc(%char / 4096))),10,16,2),%,$base($calc(128 + ($int($calc(%char / 64)) % 64)),10,16,2),%,$base($calc(128 + (%char % 64)),10,16,2))
    elseif (%char < 2097152) var %str = $+(%str,%,$base($calc(240 + $int($calc(%char / 262144))),10,16,2),%,$base($calc(128 + ($int($calc(%char / 4096)) % 64)),10,16,2),%,$base($calc(128 + ($int($calc(%char / 64)) % 64)),10,16,2),%,$base($calc(128 + (%char % 64)),10,16,2))
  }
  return %str
}