This post is part of a series based on a presentation I gave at Cheppers on March 20, 2019.

PHP quiz #3 - operator associativity

June 16, 2019

Operators are the building blocks of computer programs. You might think you know them, but the devil is in the details. After all these years, I still find myself revisiting the precedence table every once in a while.

Question

What will this code output?

<?php

$x = 2;
$y = 4;
$z = 6;

if ($z > $y > $x) {
  echo 'true';
} else {
  echo 'false';
}

A
```
Syntax error
```
B
```
true
```
C
```
false
```

Answer

Show the answer

A

Syntax error

Explanation

Operator associativity

Operator associativity decides how operators of equal precedence are grouped.

operator	associativity	example	result
`+` (addition)	left	`1 + 2 + 3`	`(1 + 2) + 3`
`=` (assignment)	right	`$a = $b = $c`	`$a = ($b = $c)`

Non-associative operators

In PHP, however, comparison operators are non-associative. They cannot be used next to each other.

The expression $z > $y > $x is illegal.

operator	associativity	example	result
`>` (greater-than)	non-associative	`$z > $y > $x`	syntax error

Other languages

Most programming languages do not allow the chaining of relational operators. The desired result is usually achieved with something like $z > $y && $y > $x.

A notable exception is Python. It evaluates chained relational operators the way someone less scarred by programming would expect.

x = 2
y = 4
z = 6

if z > y > x:
  print('true')
else:
  print('false')

# true

Awesome!

Conclusion

There is a reason why you've never seen comparison operators chained like this in PHP code. It's because it's not legal.

Exam questions often try to trick you like this. When I notice something fishy and the problem isn't completely obvious, as a rule of thumb, I always consider the 'syntax error' answers first. Trust your intuition!

This post was inspired by Upwork's incorrect PHP interview questions (question #10).

Bendegúz Csirmaz