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The PHP manual text and comments are covered by the Creative Commons Attribution 3.0 License © the PHP Documentation Group - Impressum - mail("TO:Reinhard Neidl",...)
Ein Operator ist etwas das Sie mit einem oder mehreren Werten füttern (oder Ausdrücken, um im Programmierjargon zu sprechen) und Sie erhalten als Ergebnis einen anderen Wert (damit wird diese Konstruktion selbst zu einem Ausdruck). Als Eselsbrücke können Sie sich Operatoren als Funktionen oder Konstrukte vorstellen, die Ihnen einen Wert zurück liefern (ähnlich print) und alles, was Ihnen keinen Wert zurück liefert (ähnlich echo) als irgend etwas Anderes.
Es gibt drei Arten von Opratoren. Als erstes gibt es den unären Operator, der nur mit einem Wert umgehen kann, zum Beispiel ! (der Verneinungsoperator) oder ++ (der Inkrementoperator). Die zweite Gruppe sind die sogenannten binären Operatoren; diese Gruppe enthält die meisten Operatoren, die PHP unterstützt. Eine Liste dieser Operatoren finden Sie weiter unten im Abschnitt Operator-Rangfolge .
Die dritte Gruppe bildet der ternäre Operator : ?:. Dieser sollte eher benutzt werden um abhängig von einem dritten Ausdruck eine Auswahl zwischen zwei Ausdrücken zu treffen, als zwischen zwei Sätzen oder Pfaden der Programmausführung zu wählen. Übrigens ist es eine sehr gute Idee ternäre Ausdrücke in Klammern zu setzen.
Method chaining is read left to right (left associative):
<?php
class Test_Method_Chain
{
public function One()
{
echo "One" . PHP_EOL;
return $this;
}
public function Two()
{
echo "Two" . PHP_EOL;
return $this;
}
public function Three()
{
echo "Three" . PHP_EOL;
return $this;
}
}
$test = new Test_Method_Chain();
$test->One()->Two()->Three();
/* Ouputs:
One
Two
Three
*/
?>
The -> operator, not listed above, is called "object operator" (T_OBJECT_OPERATOR).
The variable symbol '$' should be considered as the highest-precedence operator, so that the variable variables such as $$a[0] won't confuse the parser. [http://www.php.net/manual/en/language.variables.variable.php]
Note that in php the ternary operator ?: has a left associativity unlike in C and C++ where it has right associativity.
You cannot write code like this (as you may have accustomed to in C/C++):
<?php
$a = 2;
echo (
$a == 1 ? 'one' :
$a == 2 ? 'two' :
$a == 3 ? 'three' :
$a == 4 ? 'four' : 'other');
echo "\n";
// prints 'four'
?>
You need to add brackets to get the results you want:
<?
$a = 2;
echo ($a == 1 ? 'one' :
($a == 2 ? 'two' :
($a == 3 ? 'three' :
($a == 4 ? 'four' : 'other') ) ) );
echo "\n";
//prints 'two'
?>
<?php
$result1 = 7 + 8 * 9/3 -4;
$result2 = 7 + 8 * (9/3 -4);
$result3 =(7 + 8)* 9/3 -4;
echo "Result1 for 7 + 8 * 9/3 -4 = $result1 Result2 for 7 + 8 * (9/3 -4) = $result2 and Result3 (7 + 8)* 9/3 -4 = $result3 "
/*
which gives results as under
Result1 for 7 + 8 * 9/3 -4 = 27 Result2 for 7 + 8 * (9/3 -4) = -1 and Result3 (7 + 8)* 9/3 -4 = 41
Execution Order is 1) expression in brackets 2) division 3) multiplication 4) addition and 5) subtraction
*/
?>
This is very common problem: set one variable to another, if it is not empty. If it is, set it to something else.
For example: set $bar to $foo, if $foo is empty, set $bar to "undefined";
if(!empty($foo)) $bar= $foo; else $bar= "undefined";
OR operator can shorten it:
$bar= @$foo or $bar= "undefined";
The scope resolution operator ::, which is missing from the list above, has higher precedence than [], and lower precedence than 'new'. This means that self::$array[$var] works as expected.
In response to mathiasrav at gmail dot com:
The reason for that behavior is the parentheses. From the description:
"Parentheses may be used to force precedence, if necessary. For instance: (1 + 5) * 3 evaluates to 18."
So the order of operations says that even though the equality operator has higher precedence, the parentheses in your statement force the assignment operator to a higher precedence than the equality operator.
That said, it still doesn't work the way you expect it to. Neither way works, for these reasons:
<?php
if ( $a != ($a = $b) )
?>
Order of operations says to do the parentheses first. So you end up with:
<?php
$a = $b;
if ( $a != $a )
?>
Which is obviously going to be false. Without the parentheses:
<?php
if ( $a != $a = $b )
?>
Order of operations says to do the inequality first, then the assignment, so you have:
<?php
if ( $a != $a );
$a = $b;
?>
Which again is not what you expected, and again will always be false. But because you are only working with values of 0 and 1, you can make use of the XOR operator:
<?php
if ( $a ^= $b )
?>
This will only be true if 1) $a is 0 and $b is 1, or 2) $a is 1 and $b is 0. That is precisely what you wanted, and it even does the assignment the way you expected it to.
<?php
foreach ($ourstring as $c) {
if ($bold ^= $c['bold']) $resstring .= bold;
if ($underline ^= $c['underline']) $resstring .= underline;
$resstring .= $c[0];
}
?>
That code now works and produces the output you expected.
Simple POST and PRE incremnt sample:
<?php
$b = 5;
$a = ( ( ++$b ) > 5 ); // Pre-increment test
echo (int)$a;
$b = 5;
$a = ( ( $b++ ) > 5 ); // Post-increment test
echo (int)$a;
?>
This will output 10, because of the difference in post- and pre-increment operations
A quick note to any C developers out there, assignment expressions are not interpreted as you may expect - take the following code ;-
<?php
$a=array(1,2,3);
$b=array(4,5,6);
$c=1;
$a[$c++]=$b[$c++];
print_r( $a ) ;
?>
This will output;-
Array ( [0] => 1 [1] => 6 [2] => 3 )
as if the code said;-
$a[1]=$b[2];
Under a C compiler the result is;-
Array ( [0] => 1 [1] => 5 [2] => 3 )
as if the code said;-
$a[1]=$b[1];
It would appear that in php the increment in the left side of the assignment is processed prior to processing the right side of the assignment, whereas in C, neither increment occurs until after the assignment.
of course this should be clear, but i think it has to be mentioned espacially:
AND is not the same like &&
for example:
<?php $a && $b || $c; ?>
is not the same like
<?php $a AND $b || $c; ?>
the first thing is
(a and b) or c
the second
a and (b or c)
'cause || has got a higher priority than and, but less than &&
of course, using always [ && and || ] or [ AND and OR ] would be okay, but than you should at least respect the following:
<?php $a = $b && $c; ?>
<?php $a = $b AND $c; ?>
the first code will set $a to the result of the comparison $b with $c, both have to be true, while the second code line will set $a like $b and THAN - after that - compare the success of this with the value of $c
maybe usefull for some tricky coding and helpfull to prevent bugs :D
greetz, Warhog
Other Language books' operator precedence section usually include "(" and ")" - with exception of a Perl book that I have. (In PHP "{" and "}" should also be considered also). However, PHP Manual is not listed "(" and ")" in precedence list. It looks like "(" and ")" has higher precedence as it should be.
Note: If you write following code, you would need "()" to get expected value.
<?php
$bar = true;
$str = "TEST". ($bar ? 'true' : 'false') ."TEST";
?>
Without "(" and ")" you will get only "true" in $str.
(PHP4.0.4pl1/Apache DSO/Linux, PHP4.0.5RC1/Apache DSO/W2K Server)
It's due to precedence, probably.
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