Using braces with dynamic variable names in PHP

Wrap them in {}:

${"file" . $i} = file($filelist[$i]);

Working Example

Using ${} is a way to create dynamic variables, simple example:

${'a' . 'b'} = 'hello there';
echo $ab; // hello there

I do this quite often on results returned from a query..

e.g.

// $MyQueryResult is an array of results from a query

foreach ($MyQueryResult as $key=>$value)
{
   ${$key}=$value;
}

Now I can just use $MyFieldname (which is easier in echo statements etc) rather than $MyQueryResult['MyFieldname']

Yep, it's probably lazy, but I've never had any problems.


Overview

In PHP, you can just put an extra $ in front of a variable to make it a dynamic variable :

$$variableName = $value;

While I wouldn't recommend it, you could even chain this behavior :

$$$$$$$$DoNotTryThisAtHomeKids = $value;

You can but are not forced to put $variableName between {} :

${$variableName} = $value;

Using {} is only mandatory when the name of your variable is itself a composition of multiple values, like this :

${$variableNamePart1 . $variableNamePart2} = $value;

It is nevertheless recommended to always use {}, because it's more readable.

Differences between PHP5 and PHP7

Another reason to always use {}, is that PHP5 and PHP7 have a slightly different way of dealing with dynamic variables, which results in a different outcome in some cases.

In PHP7, dynamic variables, properties, and methods will now be evaluated strictly in left-to-right order, as opposed to the mix of special cases in PHP5. The examples below show how the order of evaluation has changed.

Case 1 : $$foo['bar']['baz']

  • PHP5 interpetation : ${$foo['bar']['baz']}
  • PHP7 interpetation : ${$foo}['bar']['baz']

Case 2 : $foo->$bar['baz']

  • PHP5 interpetation : $foo->{$bar['baz']}
  • PHP7 interpetation : $foo->{$bar}['baz']

Case 3 : $foo->$bar['baz']()

  • PHP5 interpetation : $foo->{$bar['baz']}()
  • PHP7 interpetation : $foo->{$bar}['baz']()

Case 4 : Foo::$bar['baz']()

  • PHP5 interpetation : Foo::{$bar['baz']}()
  • PHP7 interpetation : Foo::{$bar}['baz']()

Try using {} instead of ():

${"file".$i} = file($filelist[$i]);

Tags:

Php

Variables

Dynamic

Dynamic Variables

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