PHP 与 JavaScript 的运算符优先级差异

两者优先级大部分都一样，比较（comparision）运算和赋值（assignment）运算有细微的差别。比较运算符有 >, <, >= 等等，赋值运算符有 =, +=, *= 等等。
JS 里比较运算符比赋值运算符优先级高。于是 foo = 1 < 2 结果 foo = false；
PHP 里反过来，赋值运算符比比较运算符优先级高。于是 foo = 1 < 2 结果 foo = 1, 表达式为 false。为达到与上面同样的结果，需加上小括号
foo = (1 < 2)

附：PHP与JavaScript完整的运算符优先级

JS

The following table is ordered from highest (19) to lowest (0) precedence.

Precedence	Operator type	Associativity	Individual operators
19	Grouping	n/a	( … )
18	Member Access	left-to-right	… . …
Computed Member Access	left-to-right	… [ … ]
new (with argument list)	n/a	new … ( … )
17	Function Call	left-to-right	… ( … )
new (without argument list)	right-to-left	new …
16	Postfix Increment	n/a	… ++
Postfix Decrement	n/a	… --
15	Logical NOT	right-to-left	! …
Bitwise NOT	right-to-left	~ …
Unary Plus	right-to-left	+ …
Unary Negation	right-to-left	- …
Prefix Increment	right-to-left	++ …
Prefix Decrement	right-to-left	-- …
typeof	right-to-left	typeof …
void	right-to-left	void …
delete	right-to-left	delete …
14	Exponentiation	right-to-left	… ** …
Multiplication	left-to-right	… * …
Division	left-to-right	… / …
Remainder	left-to-right	… % …
13	Addition	left-to-right	… + …
Subtraction	left-to-right	… - …
12	Bitwise Left Shift	left-to-right	… << …
Bitwise Right Shift	left-to-right	… >> …
Bitwise Unsigned Right Shift	left-to-right	… >>> …
11	Less Than	left-to-right	… < …
Less Than Or Equal	left-to-right	… <= …
Greater Than	left-to-right	… > …
Greater Than Or Equal	left-to-right	… >= …
in	left-to-right	… in …
instanceof	left-to-right	… instanceof …
10	Equality	left-to-right	… == …
Inequality	left-to-right	… != …
Strict Equality	left-to-right	… === …
Strict Inequality	left-to-right	… !== …
9	Bitwise AND	left-to-right	… & …
8	Bitwise XOR	left-to-right	… ^ …
7	Bitwise OR	left-to-right	… | …
6	Logical AND	left-to-right	… && …
5	Logical OR	left-to-right	… || …
4	Conditional	right-to-left	… ? … : …
3	Assignment	right-to-left	… = …
… += …
… -= …
… **= …
… *= …
… /= …
… %= …
… <<= …
… >>= …
… >>>= …
… &= …
… ^= …
… |= …
2	yield	right-to-left	yield …
1	Spread	n/a	... …
0

PHP

Operator Precedence

Associativity	Operators	Additional Information
non-associative	clone new	clone and new
left	[	array()
right	**	arithmetic
right	++ -- ~ (int) (float) (string) (array) (object) (bool) @	types and increment/decrement
non-associative	instanceof	types
right	!	logical
left	* / %	arithmetic
left	+ - .	arithmetic and string
left	<< >>	bitwise
non-associative	< <= > >=	comparison
non-associative	== != === !== <> <=>	comparison
left	&	bitwise and references
left	^	bitwise
left	|	bitwise
left	&&	logical
left	||	logical
right	??	comparison
left	? :	ternary
right	= += -= *= **= /= .= %= &= |= ^= <<= >>= =>	assignment
left	and	logical
left	xor	logical
left	or	logical