(PHP 4 >= 4.2.0, PHP 5)
fmod — Returns the floating point remainder (modulo) of the division of the arguments
$x
    , float $y
    )
   Returns the floating point remainder of dividing the dividend
   (x) by the divisor (y).
   The remainder (r) is defined as: x = i * y + r, for 
   some integer i. If y is
   non-zero, r has the same sign as
   x and a magnitude less than the magnitude of
   y.
  
xThe dividend
yThe divisor
   The floating point remainder of 
   x/y
  
Example #1 Using fmod()
<?php
$x = 5.7;
$y = 1.3;
$r = fmod($x, $y);
// $r equals 0.5, because 4 * 1.3 + 0.5 = 5.7
?>