Question

I came across some code today where a string is compared to two values at the same time. I’ve never seen this before – will this work? Can someone explain it to me?

$foo = 'date';

if ($foo == ('date' || 'datetime')) {
  echo "Hello world";
}

Answer 1

That won’t work. Write if ($foo == 'date' || $foo == 'datetime').

Not only won’t || work for selecting from a set, but also you use used a single =, which is for assignment rather than comparison.

In this case, the constant strings are compared using the boolean or operator. To do that, they are both converted to boolean. Since they are non-empty strings, they evaluate to true. true or true returns true, which is assigned to $foo which is compared to $foo. That comparison will always be true if $foo is ‘date’ or ‘datetime’ or about any other non-empty string.

So, whatever the previous value of $foo was, or even if it wasn’t assigned at all, the if-expression always evaluates to true, so you always get the echo, and $foo will always be true afterwards.