php 简写 if

Question

I'm struggling to understand why the following code is echoing out 'FOO2' when i'm expecting 'FOO1'

$tmp = 'foo1';
echo $tmp == 'foo1' ? 'FOO1' : $tmp == 'foo2' ? 'FOO2' : 'NO FOO';

Answer 1

$tmp = 'foo1';
echo $tmp == 'foo1' ? 'FOO1' : ($tmp == 'foo2' ? 'FOO2' : 'NO FOO');

Answer 2

basically PHP breaks this down to:

$tmp = 'foo1';
echo ($tmp == 'foo1' ? 'FOO1' : $tmp == 'foo2') ? 'FOO2' : 'NO FOO';

the part in parentheses will return FOO1 which evaluates to TRUE so the second conditional statement essentially is TRUE? 'FOO2': 'NO FOO'; TRUE? 'FOO2': 'NO FOO'; – which in turn always evaluates to 'FOO2'

Note: This is different from C ternary operator associativity

Answer 3

$tmp = 'foo1';
if($tmp == 'foo1') echo 'FOO1';
else if($tmp == 'foo2') echo 'FOO2';

As you have just found out, ternary operators are a minefield of confusion, especially when you try to nest stack them. Don't do it!

Edit:
The PHP manual also recommends not stacking ternary operators:-

It is recommended that you avoid "stacking" ternary expressions. PHP's behaviour when using more than one ternary operator within a single statement is non-obvious:

See example 3 on this page of the PHP Manual

Answer 4

$tmp = 'foo1';
echo $tmp == 'foo1' ? 'FOO1' : ($tmp == 'foo2' ? 'FOO2' : 'NO FOO');