PHP中奇怪的逻辑行为 - 这是正常的吗？

Question

Can anyone figure out why this might happen in PHP (am using v5.4):

$value = 0;
$existing_value = "Unknown";

if ($value == $existing_value) {
    echo "$value == $existing_value";
} else {
    echo "$value != $existing_value";
}

This outputs as 0 == Unknown

Interestingly, $value = "0" (i.e. set as a string), evaluates to be false

Is this a known behaviour? Have I missed something in the documentation on this? Debugging this was driving me crazy earlier today!

Thanks for your help in advance...

Answer 1

This is caused by the automatic type casting, PHP uses.

When comparing an int value with a string using just ==, the string will be casted to an int, which in your case results in a 0 and hence a true evaluation.

See the respective PHP documentation for more information.

To circumvent this, you could use === instead of ==. The former includes a type check, which will make your condition evaluate to false:

$value = 0;
$existing_value = "Unknown";

if ($value === $existing_value) {
    echo "$value === $existing_value";
} else {
    echo "$value !== $existing_value";
}