OPERADORES ARITMETICOS EN PHP
$a + $b Addition Sum of $a and $b.
$a - $b Subtraction Difference of $a and $b.
$a * $b Multiplication Product of $a and $b.
$a / $b Division Quotient of $a and $b.
$a % $b Modulus Remainder of $a divided by $b.
<?
$a=$a + $b;
?>
o checate en estas direcciones los manuales
http://www.webestilo.com/php http://www.desarrolloweb.com/php