为什么if（陈述）; 编译并运行？ [重复]

Question

This question already has an answer here:

• What does if(); do, where the semi-colon is right after the parentheses? 2 answers

I've always been wondering about this since I began learning PHP. As many other new-beginners, I placed semicolons all over the place, and for some reason, placing them after an if statement compiles and runs, but not like one would expect.

Example:

<?php
$foo = "bar";

if ($foo == "bar") // Correct way
    echo "OK";

if ($foo == "bar"); // Incorrect way
    echo "OK";

if ($foo == "derp"); // Incorrect way again
    echo "NO";
?>

This will output:

OKOKNO

Why is this? What is the intended usage for this? Just a twerk left in the code that for some reason has never been fixed? It looks like the compiler just skips the entire if-statement all together.

</div>

Answer 1

Because the PHP interpreter is looking for one statement after the if(...) to execute if the condition is evaluated to true. Appending the semicolon tells the interpreter that you're done with the statement. You've effectively told PHP to execute a no-op statement when the condition holds true. The echo isn't even part of the if-statement anymore.

It's equivalent to writing

if($foo == "bar"){
}
echo "OK";