String Comparison to Integer: Uncommon Phenomena Explained

In PHP, comparing strings to integers can sometimes yield quirky results. This confusion stems from PHP's loose comparison behavior, specifically when comparing strings to the value 0.

Consider the following code:

$test1 = "d85d1d81b25614a3504a3d5601a9cb2e";
$test2 = "3581169b064f71be1630b321d3ca318f";

if ($test1 == 0)
  echo "Test 1 is Equal!?";
if ($test2 == 0)
  echo "Test 2 is Equal!?";

Surprisingly, the output of this code is:

Test 1 is Equal!?

Why does this occur?

According to the PHP documentation on string conversion to numbers, when a string is evaluated in a numeric context (such as a comparison), it is converted to a float or integer based on its content. If the string contains characters like '.', 'e', or 'E', it is treated as a float. Otherwise, it is considered an integer.

In the case of $test1, the string does not contain any numeric characters and thus is converted to the integer value 0. This is why the comparison $test1 == 0 evaluates to true.

Why does $test2 not work?

$test2 also starts with non-numeric characters, so why doesn't it also equal 0? This is because the string contains the character '3' at the start. According to the PHP documentation, a valid number starts with a sign (optional) followed by digits. Thus, $test2 is converted to a float with a value of 0, and the comparison $test2 == 0 evaluates to false.