i think i didn't understand but i try...
<? $yourVar = "FOO"; ?>
<input type="text" value="<?echo $yourVar;?>" name="da">
or if you want force some value when you submit...
<script language="javascript">
function zzz() {
// force some value if empty
if(document.formi.da.value.length == 0) {
document.formi.da.value = <? echo $yourVar;?>
}
document.formi.submit();
}
</script>
<forn name="formi" action="act.php">
<input type="text" name="da" value="">
<input type="button" name="xx" value="SUBMIT" onclick="javascript:zzz();">
</form>