Workaround for PHP syntax 0 not equal to null empty

Today do information collection page encountered a strange problem, study for a long time to find the root cause of the problem, it is a bit strange indeed.
(the younger brother often uses the C #, Java and other languages, PHP only occasionally in use, not research very deep). Please senior PHP programmer don't laughed at

The problem is this: in PHP statements (0!=null && 0 >=0), the returned result is empty, which is really very strange.

Experiment Summary:
The PHP statements are as follows:

$index = 0;

echo "A:". $index. "
"; 0

echo "B:". ($index!=null && $index >=0). "
";//
echo "C:". (Isset ($index) && $index >=0). "
";//1

echo "D:". (0!=null). "
";//

Results:

a:0
B:
c:1
D:
Visible: The result of 0!=null is empty. To make the results correct, you can modify the $index!=null to Isset ($index)

if (Isset ($index) && $index >=0))
{
echo "contains.";
}else{
echo "does not contain.";
}

This is very strange, finally solved. Mark, please.

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