Does PHP have short-circuit evaluation?

Question

Given the following code:

if (is_valid($string) && up_to_length($string) && file_exists($file))  {     ...... } 

If is_valid($string) returns false, does the php interpreter still check later conditions, like up_to_length($string)?
If so, then why does it do extra work when it doesn't have to?

Answer 1

Yes, the PHP interpreter is "lazy", meaning it will do the minimum number of comparisons possible to evaluate conditions.

If you want to verify that, try this:

function saySomething() {     echo 'hi!';     return true; }  if (false && saySomething()) {     echo 'statement evaluated to true'; } 

Answer 2

Yes, it does. Here's a little trick that relies on short-circuit evaluation. Sometimes you might have a small if statement that you'd prefer to write as a ternary, e.g.:

    if ($confirmed) {         $answer = 'Yes';     } else {         $answer = 'No';     } 

Can be re-written as:

   $answer = $confirmed ? 'Yes' : 'No'; 

But then what if the yes block also required some function to be run?

    if ($confirmed) {         do_something();          $answer = 'Yes';     } else {         $answer = 'No';     } 

Well, rewriting as ternary is still possible, because of short-circuit evaluation:

    $answer = $confirmed && (do_something() || true) ? 'Yes' : 'No'; 

In this case the expression (do_something() || true) does nothing to alter the overall outcome of the ternary, but ensures that the ternary condition stays true, ignoring the return value of do_something().