$i = 1; 
$z = 2; 
<input name="anzahlTage[<?php echo $wk->wid;?>]" type="text" value="<?php echo $wk->anzahlTage;?>" size="2" id="<?php echo $i++;?>" class="form" onkeyup="document.getElementById('<?php echo $z++;?>').value = this.value"/> 

<script type="text/javascript"><!-- 
function nachuntenfuellen(o){ 
    var wert = o.value; 
    var formName = o.form.name 
    var start = (o.name) * 1; 
    var stop = (o.form[o.form.length - 1].name) * 1; 
    for(var z = start; z < stop; z++){ 
        document[formName][''+z].value = wert; 
    } 
} 
// --!> 
</script> 
Nach unten ausfüllen 
<form name="formular"> 
<input name="1" onkeyup="nachuntenfuellen(this)"><br> 
<input name="2" onkeyup="nachuntenfuellen(this)"><br> 
<input name="3" onkeyup="nachuntenfuellen(this)"><br> 
<input name="4" onkeyup="nachuntenfuellen(this)"><br> 
<input name="5" onkeyup="nachuntenfuellen(this)"><br> 
<input name="6" onkeyup="nachuntenfuellen(this)"><br> 
<input name="7"  onkeyup="nachuntenfuellen(this)"><br> 
</form> 

 <input name="anzahlTage[<?php echo $wk->wid;?>]" type="text" value="<?php echo $wk->anzahlTage;?>" size="2" id="" class="form" tabindex="1"/>