How does work if / else? - php

Question

I have $first and $second. They can have value 0 or 1.
if ( $first AND $second ) {
// True
} else {
// False
}
My mind (and Google search) tells me, that the result is true only when $first == 1 and $second == 0 or vice versa. But the result is true when both of this variables are 1.
I don't understand how does it works.

Answer 1

Your Google searches have failed you. PHP's type juggling means that a 1 is equivalent to TRUE and 0 is equivalent to FALSE. (See also type comparisons). So if both values are 1 then that if statement evaluates to TRUE. If one or both values are 0, it evaluates to FALSE.
<?php
$one = 1;
$zero = 0;
if ($one && $one) {
echo "true\n";
}
else {
echo "false\n";
}
if ($zero && $zero) {
echo "true\n";
}
else {
echo "false\n";
}
if ($one && $zero) {
echo "true\n";
}
else {
echo "false\n";
}
if ($zero && $one) {
echo "true\n";
}
else {
echo "false\n";
}
Program Output
true
false
false
false
Demo

Answer 2

In PHP all values are either "truthy" or "falsy" in expressions.
If a value contains something then it can be said to be truthy. So, values such as 1, "one", [1,2,3] or true all "contain" something and will be interpreted as truthy.
Values that are zeroed or in some way empty are falsy. E.g. 0, "", [] and false.
There is a table of how values are interpreted in the PHP documentation.
You can also just experiment, and output it to your website:
var_dump(1 and 0);