Difference between 'or' and '||'?

Question

$conn = mysql_connect($db_host,$db_user,$db_password) || die('Connection to mysql failed');
mysql_close($conn);

On executing this code, the following warning was displayed.

PHP Warning: mysql_close() expects parameter 1 to be resource, boolean given in script.php on line 45

$conn = mysql_connect($db_host,$db_user,$db_password) or die('Connection to mysql failed');
mysql_close($conn);

No Warning Now!?

Answer 1

&& and || are logical operators -- they're for Boolean conditional statements. As @towr and @ChrisHayes point you can use and and or in place of the && and || syntax, albeit at a lower precedence than most other operators.

or in this context, however, is completely different -- it's part of the control flow (see section 2.5.7: exit and return).

Boolean condition:

if ($foo == $bar || $bar != $bob)

Control flow:

mysql etc... or die();

You're receiving the error "PHP Warning: mysql_close() expects parameter 1 to be resource..." because you're not actually writing correct syntax, thus breaking your mysql_connect statement.

Answer 2

and and or have higher lower precedence than && and ||. To be more exact && and || have higher precedence than assignment operator ( = ) while and and or have lower.

http://www.php.net/manual/en/language.operators.precedence.php

Usually it doesn't make a difference, but there are cases when not knowing about this difference can cause some unexpected behaviour. See examples here:

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

EDIT(suggested by @towr):

Applied to the question at hand, this means that in the first case we assign to $conn the value mysql_connect(....) || die('....'),because || has a higher precendence than =. The problem here is that $conn now is a boolean, and not a resource.

In the second case we OR the expressions $conn = mysql_connect(....) and die('....'), because = has a higher precendence than OR. We do nothing with the boolean value, and $conn is simply the resource we assigned to it in the first expression (if it didn't fail).