<?php

define ("DBL_MANT_DIG", 16);

/*
 * Checks if value is a valid step.
 *
 * This is based on the number/range verification methods of webkit.
 *
 * @see http://codesearch.google.com/#OAMlx_jo-ck/src/third_party/WebKit/Source/WebCore/html\numberInputType.cpp&type=cs&l=129
 *
 * @param $value
 *   The value that needs to be checked.
 * @param $step
 *   The step scale factor for the widget.
 * @param $min
 *   The min value of the widget.
 *
 * @return
 *   TRUE if value is invalid
 *   FALSE if value is valid
 */
function stepMismatch($value, $step, $min = 0.0) {
  $doubleValue = (double)$value;
  $doubleValue = abs($doubleValue - $min);

  // double's fractional part size is DBL_MANT_DIG-bit. If the current value
  // is greater than step*2^DBL_MANT_DIG, the following computation for
  // remainder makes no sense.
  if ($doubleValue / pow(2.0, DBL_MANT_DIG) > $step) {
    return false;      
  }
  // The computation follows HTML5 4.10.7.2.10 `The step attribute' :
  // ... that number subtracted from the step base is not an integral multiple
  // of the allowed value step, the element is suffering from a step mismatch.
  (double) $remainder = abs($doubleValue - $step * round($doubleValue / $step));
  // Accepts erros in lower fractional part which IEEE 754 single-precision
  // can't represent.
  $computedAcceptableError = (double)($step / pow(2.0, DBL_MANT_DIG));

  return $computedAcceptableError < $remainder && $remainder < ($step - $computedAcceptableError);
}


$value = 609.090909;
$step = 9.09090909;

if (stepMismatch($value, $step)) {
  echo "INVALID\n"; 
}
else {
  echo "VALID\n"; // Print Valid
}

$value = 1.2;
$step = 0.5;

if (stepMismatch($value, $step)) {
  echo "INVALID\n"; // Print Invalid
}

else {
  echo "VALID\n";
}

$value = 0.9411764;
$step = 0.00392156863;

if (stepMismatch($value, $step)) {
  echo "INVALID\n"; // Print Invalid
}

else {
  echo "VALID\n";
}

$value = 0.9411764712;
$step = 0.00392156863;

if (stepMismatch($value, $step)) {
  echo "INVALID\n"; 
}

else {
  echo "VALID\n"; // Print valid
}

$value = 0.1;
$step = 0.2;

if (stepMismatch($value, $step)) {
  echo "INVALID\n";  // Print Invalid
}
else {
  echo "VALID\n";
}
  

$value = 0.1;
$step = 0.2;
$min = -0.3;

if (stepMismatch($value, $step, $min)) {
  echo "INVALID\n"; 
}

else {
  echo "VALID\n"; // Print valid
}

?>