I have recreated this problem in both PHP 3.0.16 and 4.0 RC2.  When using an array of checkboxes that had a zero based value, the following script had variable and erratic results. When the commented out for{} construct that start at 1 are added back and comment out the for{} that uses 0, it works fine.  After reviewing the Open Bug Report list, I have found that Bug ID #4433 may be having the same problem.

Script to recreate problem:
----------------------------

<?php

  echo "<form action='$PHP_SELF' method='post'>\n";

		$NumChecks = count($chkArray);
		echo "The number of Checks in the array is: $NumChecks<BR>\n";
		if ($NumChecks > 0)
		{
			for ($y=0; $y<$NumChecks; ++$y)
			//for ($y=1; $y<=10; ++$y)
			{
				echo "\$chkArray[$y] is $chkArray[$y]<BR>\n";
			}
		}
		else 
		{
			echo "\$chkArray is empty<BR>\n";
		} 

		$NumHide = count($HideArray);
		echo "The number of Hidden items in the array is: $NumHide<BR>\n";
		If ($NumHide > 0)
		{
			for ($y=0; $y<$NumHide; ++$y)
			//for ($y=1; $y<=$NumHide; ++$y)
			{
				echo "\$HideArray[$y] is $HideArray[$y] <br>\n";
			}
		}
		else
		{
			echo "\$HideArray is empty<BR>\n";
		}


		//for( $x=1; $x<=10; ++$x)
		for( $x=0; $x<10; ++$x)
		{

			$strChecked = "";
			if ( $chkArray[$x] == $x ) {	$strChecked = " checked "; }

			$HideVal = $x+100;
      
			echo "<input type='checkbox' name='chkArray[$x]'  value=$x".$strChecked."><br>\n";
			echo "<input type='hidden'   name='HideArray[$x]' value=$HideVal>\n";
		}


		echo "<input type='submit' name='GetData' value='Submit'>";

  echo "</form>";

?>