php “如果”条件之谜

Question

I am running into a funny problem with a mischievous "if" condition:

$condition1="53==56";
$condition2="53==57";
$condition3="53==58";
$condition=$condition1."||".$condition2."||".$condition3;
if($condition)
{
    echo "blah";
}
else
{
    echo "foo";
}

Why does the if condition pass? Why does php echo "blah"? What do I do to make php evaluate the "if" statement and print "foo"?

Answer 1

The problem here is that you're putting your expressions in strings!

Your $condition1 , $condition2 , and $condition3 variables contain strings, and not the result of an expression, and the same goes for your $condition variable which will be a string that looks like 53==56||53==57||53==58 . When PHP evaluates a string it considers it true if it is not empty and not equal to 0 , so your script will output blah .

To fix this you just need to take your expressions out of the strings. It should look like this:

$condition1 = 53 == 56; // false
$condition2 = 53 == 57; // false
$condition3 = 53 == 58; // false
$condition = $condition1 || $condition2 || $condition3; // false || false || false = false

if ($condition) {
    echo 'blah';
} else {
    echo 'foo'; // This will be output
}

Answer 2

You're evaluating strings as booleans; they'll aways be true (except the strings "" and "0" . Get rid of almost all of the quotes in your program.

Answer 3

Those aren't conditions, they're strings.

$condition1=53==56;
$condition2=53==57;
$condition3=53==58;
$condition=$condition1 || $condition2 || $condition3;
if($condition)
{
    echo "blah";
}
else
{
    echo "foo";
}

Answer 4

Because you're not checking those variables, it's saying if (String) will always return true. (unless "" )

You should be doing:

if(53==56 || 53==57 || 53==58)
{
    echo "blah";
}
else
{
    echo "foo";
}

Answer 5

All $condition* variables will evaluate to true. This is how PHP sees it:

if("53==56" || "53==57" || "53==58")

What you want is this:

$condition1 = 53==56;
$condition2 = 53==57;
$condition3 = 53==58;

Answer 6

It's because you're evaluating a string, and strings other than empty strings evaluate to true.

Answer 7

You are concatting a string together, a non-empty string equals TRUE in php.

Answer 8

Because when the if passes, $condition is a string (a concatenation of) containing the text of your conditions. Try using if(eval($condition)) .

Answer 9

String always evaluate to true if its not empty
And btw php make implicit conversion to boolean