Clarification on how PHP outputs true and false values [duplicate]

What you are seeing is the result of "type juggling", where you have a variable which is a boolean, and you're using it somewhere you need a string.

The rules for that are described in the "Strings" section of the manual:

A bool true value is converted to the string "1". bool false is converted to "" (the empty string). This allows conversion back and forth between bool and string values.

Your first example is a bit confusing, because you haven't offset the value with a newline or other separator, and your strings begin "falsevar" and "truevar", making it really hard to read the output. Here's a clearer version:

$falsevar = false;
$truevar = true;
echo "Variable set to false is [" . $falsevar . "]\n";
echo "Variable set to true is [" . $truevar. "]\n";

As described in the manual, this results in:

Variable set to false is []
Variable set to true is [1]

For debugging, it's generally more useful to use a function like var_dump, which gives you a description of a value. In your second example, you could write this:

$prompt = "I would like a sandwich to eat";
$myarray = array("pizza", "hamburger", "hot dog");
$wantsItem = false;
foreach ($myarray as $needle) {
    if (strpos($prompt, $needle) != false) {
        echo "Match found: {$needle}\n";
        $wantsItem = true;
    }
}
var_dump($wantsItem);

That gives the output:

bool(false)

Aside, as Robert Mihai Ionas pointed out, you have a subtle bug in your code, unrelated to the question: strpos returns 0 for "matches at the start", and false for "does not match", and this causes a different type-juggling problem: both are "false-y values". So as the strpos manual page notes, you should use strpos($prompt, $needle) !== false instead of strpos($prompt, $needle) != false.

Alternatively, you can use the recently-added str_contains function instead, which doesn't have this fiddly problem.