PHP conditional assignment

Question

Found an interesting piece of code in Symfony core

if ('' !== $host = $route->getHost()) {
    ...
}

The precedence of !== is higher than the = but how does it work logically? The first part is clear but the rest?

I've created a little sample but it's still not clear: sample

Do it the save way if ('' !== ($host = $route->getHost())) { — JustOnUnderMillions
– JustOnUnderMillions, Commented Mar 6, 2017 at 15:12
Possible duplicate of JS vs PHP: assignment operator precedence when used with logical-or — Shira
– Shira, Commented Mar 6, 2017 at 15:16
Look at this sandbox.onlinephpfunctions.com/code/… maybe you get wired more or get it fully ;-) But for humanreadable and savety do it in the save way. By thw way this $host = $route->getHost() can be done a line before :-) — JustOnUnderMillions
– JustOnUnderMillions, Commented Mar 6, 2017 at 15:17
I just don't get how can the $host = $route->getHost() part be executed first as the precedence is lower? — katona.abel
– katona.abel, Commented Mar 6, 2017 at 15:25

Philipp · Accepted Answer · 2017-03-06 15:48:27Z

3

The point is: The left hand side of an assignment has to be an variable! The only possible way to achieve this in your example is to evaluate the assignment first - which is what php actually does.

Adding parenthesis makes clear, what happens

'' !== $host = $route->getHost()
// is equal to
'' !== ($host = $route->getHost())
// the other way wouldn't work
// ('' != $host) = $route->getHost()

So the condition is true, if the return value of $route->getHost() is an non empty string and in each case, the return value is assigned to $host.

In addition, you could have a look a the grammer of PHP

...
variable '=' expr |
variable '=' '&' variable |
variable '=' '&' T_NEW class_name_reference | ...

If you read the operator precendence manual page carefully, you would see this notice

Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.

edited Mar 6, 2017 at 15:48

answered Mar 6, 2017 at 15:37

Philipp

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2 Comments

Jeff Puckett Over a year ago

same reason wordpress coding standards use yoda conditions

katona.abel Over a year ago

Thank you - that was my guess but could not find it in the docs

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