Operator Precedence

Like in math: 3 + 4 * 5 = 23 and not 35, in PHP you have an order for interpreting the code.

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

For operators that have the same precedence, the order is specified in the first column.
PHP also knows about Asociativity.

Exception:

 if (!$a = foo())

Even if = has lower precedence than other operators (! here) the return falue of foo() is put into $a  before the negation