Passing variable variable into closure throws a parse error?

Question

I just noticed that if you try to put a variable variable into a use list of a closure, it throws a parse error. Example code:

foreach ($array as $key => $item) {
    $$key = $item->something;
    $this->doSomething($key, function ($param) use ($item, $$key) {
        $param->foo($$key);
    });
}

The parse error is this:

Parse error: syntax error, unexpected '$', expecting '&' or variable (T_VARIABLE) in

Is there something I'm doing wrong here? Why wont it let me pass the variable variable?

If I store the variable variable's value in another variable, I can pass it normally through use, but that's not optimal for my actual case.

Anders Carstensen · Accepted Answer · 2018-12-25 19:04:24Z

1

You say you don't want to save the value in another variable. But what about saving a reference to the variable? That should be functionally equivalent to your code.

foreach ($array as $key => $item) {
    $$key = $item->something;
    $otherVar = &$$key;
    $this->doSomething($key, function ($param) use ($item, $otherVar) {
        $param->foo($otherVar);
    });
}

If this doesn't work in "your actual case", please refine the code example or explain why.

answered Dec 25, 2018 at 19:04

Anders Carstensen

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3 Comments

aborted Over a year ago

Yeah this workaround is great, but I wanted to make sure that my problem is actually to be expected or if it's just me messing something up. Why doesn't PHP allow passing variable variables like that?

Sébastien Renauld Over a year ago

@Dugi you got bitten by PHP's years of adaptative syntax. Years and years of hackish partly-implemented things (like variable-invariable syntax, i.e. $$). This solution is the closest you'll get and it's one extra allocation in total.

Anders Carstensen Over a year ago

@Dugi I can only come with an educated guess: If you were allowed to pass $$key in the use statement, you could just change $key in the function body, effectively making all variables outside of the function body available inside. That would undermine the principles of scopes.

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