PHP Learning Note Operator Precedence _php tutorial

Operator Precedence is an important point of knowledge that a programmer must understand and hold, let's take a look at the PHP operator precedence article.

Today in Lao Wang's technical manual saw a problem:

The code is as follows	Copy Code

What is the output?

This is a question that may seem simple at first glance, but in fact it is not easy to scrutinize

If the Boolean and the previous part are due to the priority problem, but if the problem is only a priority, then the result should be:

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$a = (&& $b) = 200

and the actual result, really high priority && concession to the sub-priority =, let $b = 200 first combined.

The reason for this is that PHP does not fully adhere to the definition of priority, which is also explained in the PHP manual:

Note:although = have a lower precedence than most other operators, PHP would still allow expressions similar to the FOLLOWI Ng:if (! $a = foo ()), in which case the return value of Foo () was put into $a.

Such a design, the individual does not express views, anyway in C language, so similar statements are judged to be grammatical error. PHP uses this design, which is probably the historical reason,

There are curious classmates, will want to know exactly why, before Jayeeliu netizens also asked:

Laruence Hello:

Ask a PHP operator for priority issues

The code is as follows	Copy Code
$t = = 1 && $tt = 2

The precedence of the PHP operator should be

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(($t = = 1) && $tt) = 2

This order is executed, but it should actually be

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($t = = 1) && ($tt = 2)

I don't understand a bit.

In fact, operator precedence is a means of choosing a protocol rule in the presence of ambiguity grammars, while PHP's parsing file definition does not have a statute conflict before the equals sign and T_boolean_and (&&):

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Expr_without_variable: There are hidden rules that are equivalent to T_boolean_and becoming "unary operators". | Expr T_boolean_and {zend_do_boolean_and_begin (&$1, &$2 tsrmls_cc);} exp

Finally, by the way, PHP corresponds to T_boolean_and also defines T_logical_and (and) and t_logical_or (or), both of which have lower precedence than equals sign, so there will be, a lot of PHP introductory textbook sample code in the classic:

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$result = mysql_query (*) or Die (Mysql_error ());

Similarly, you can use or to implement ternary operators (?:) Features of:

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$person = $who or $person = "laruence";

Equivalent to:

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$person = Empty ($who)? "Laruence": $who;

Combination direction	operator
Left	,
Left	Or
Left	Xor
Left	and
Right	Print
Right	= + = = *=/=. =%= &= |= ^= ~= <<= >>=
Left	? :
Left	||
Left	&&
Left	|
Left	^
Left	&
No	== != === !==
No	< <= > >=
Left	<< >>
Left	+ - .
Left	* / %
Right	! ~ + +--(int) (float) (string) (array) (object) @
Right	[
No	New

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