PHP “or” Syntax

问题:

I've seen this a lot: $fp = fopen($filepath, "w") or die(); But I can't seem to find any real documentation on this "or" syntax. It's obvious enough what it does, but can I use it anywhere? And must it be followed by die()? Are there any caveats to using or when you could use something like

if (file_exists($filepath))
   $fp = fopen($filepath,"r");
else
   myErrMessage();

I know it seems like a silly question, but I can't find any hard and fast rules for this. Thanks.

回答1:

It's a logical operator and can be used in any logical expression.

http://php.net/manual/en/language.operators.logical.php

回答2:

Let's just say that:

$result = first() || second();

will evaluate to:

if (first()) {
    $result = true;
} elseif (second()) {
    $result = true;
} else {
    $result = false;
} 

while:

$result = first() or second();

will evaluate to:

if ($result = first()) {
    // nothing
} else {
    second();
}

In other words you may consider:

$result = first() || second();

$result = (first() || second());

and:

$result = first() or second();

to be:

($result = first()) || second();

It is just matter of precedence.

回答3:

This is neat trick, inherited from some PHP predecessor, based on the fact that PHP quite smartly won't evaluate any expression following OR, if first one returned true:

function a($ret){
    echo "FOO";
    return $ret;
}
function b($ret){
    echo "BAR";
    return $ret;
}

$result = (a(true) OR b(true));

will print out only FOO, means b() weren't even executed.

回答4:

or just does a boolean comparison.

What's returned by fopen() can be treated as such a boolean value, because it returns FALSE if it fails (and a different value if it does not).

If it fails, the statement is evaluated to the right, and so the function die() is called.

回答5:

Basically it means "if the first command fails, then perform the second command." In your example, if PHP cannot open the file, it will terminate the script (die()).

回答6:

'Or' in PHP is like C-like syntax (||)

<?php 
if( ($a==1 || $a==2) && ($b==3 || $b==4) && ($c==5 || $c==6) ) { 
     //do that something here. 
} 
?>

The 'Or' you are talking about is just a trick as the following states:

Example:

$result = mysql_query('SELECT foo FROM bar', $db) or die('Query failed: ' . mysql_error($db));

The or die() trick is a very poor choice for several reasons:

1. It's not a very nice way to present the user with an error message.
2. You cannot catch the error in any way.
3. You cannot log the error.

4. You cannot control whether it should be output to the screen or not. It's okay to do that in a development environment, but certainly not in a production environment.

   5. It prevents you from doing any sort of cleanup. It just ends the script abruptly.

Reference: [enter link description here][1]

[1]: http://www.phpfreaks.com/blog/or-die-must-dieenter code here

回答7:

It can be used just like you'd use || as a logical OR http://php.net/manual/en/language.operators.logical.php

回答8:

It can be used as || but hasn't the same precedence: http://www.php.net/manual/en/language.operators.precedence.php

The precedence of an operator specifies how "tightly" it binds two expressions together. For example, in the expression 1 + 5 * 3, the answer is 16 and not 18 because the multiplication ("*") operator has a higher precedence than the addition ("+") operator. Parentheses may be used to force precedence, if necessary. For instance: (1 + 5) * 3 evaluates to 18.