in a string check routin whitch checks the validity
of characters entered in a POST field i used
while () with substr(). If the string that substr() 
parses has 0 (zero) in it, while() exits while it should
not.

e.g.
$i=0;
while ($ch=substr($field_name,$i)
{
print"$ch";
$i++;
}

if $field_name has a 0 in it, while() exits while it
should exit with another false signal other than 0 (null maybe?)