How to check if field is empty before submit

[NoPHPPhD]

I would like to make sure that "Quantity" is > zero when this is submitted.
If it is not, I would like to just redisplay the same form.
What do I have to do? Probably very simple, just not searching for the right term.... :-\

[code]
print("<FORM>\n");
print("<FORM ACTION=\"AddRecord.php\" METHOD=\"GET\">\n");
print("<INPUT TYPE=\"TEXT\" NAME=\"Quantity\">\n");
print("<INPUT TYPE=\"HIDDEN\" NAME=\"NBDno\" VALUE = '$pickedNBDno' >\n");

print("<SELECT NAME=\"Measure\">\n");
while($row = mysql_fetch_object($dbResult))
{
print("<OPTION VALUE=\"{$row->WeightID}\"");
print(">{$row->Description}\n");
}
print("</SELECT>\n");

print("<INPUT TYPE=\"submit\">\n");
print("</FORM>\n");
[/code]

[hackerkts]

You can try using [b]isset[/b], sorry I doesn't have the time to write out examples.

[trq]

You could do something like....

[code=php:0]
if (isset($_GET['Quantity']) && $_GET['Quantity'] > 0) {
  // do action.
}
[/code]

Of course this doesn't check the field until after submission (php cant), you would need to use Javascript to achieve that.

[twofivethreetwo]

This site http://www.php-mysql-tutorial.com/form-validation-with-php.php  has some info on client-side and server-side validation for forms. Might be of some help.