null and zeros

there are few variables:
$x=null;
$y=10;
$z=15;

...and few expressions similar to these below:
$result=$x+$y+$z;
$result=$x+$y$x;
...
$result=$y$x-$z;

the result of all the expressions should be null, because one of the variables is null. how to do it?

php gives in the first case 25, in the second 0. but i need the results that are nulls!

any ideas?

PHP will convert the values to their integer equivalent for math operations, and null would be converted to 0.

Like PM said, NULL==0 in PHP, so 25 is completely correct.

thanks for answer but how to solve the problem? when one of the variables is null, the result is null. i need it to be null.

Try using an IF statement:

if (($a==0) or ($b==0) or ($c==0))
{
$d = 0;
}
else
{
$d=$a+$b+$c;
};