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List:       php-general
Subject:    Re: [PHP] ${0} variable basics
From:       Robert Cummings <robert () interjinn ! com>
Date:       2014-08-25 3:45:42
Message-ID: 53FAB166.5090207 () interjinn ! com
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On 14-08-24 07:19 PM, Negin Nickparsa wrote:
> why ${0} is valid but ${0 is not valid?
> it means that curly braces evaluate what's inside them and that's why we
> cannot have ${0?
>
> variables shouldn't start from 0 so it means that it started from braces?
> if yes why I cannot have ${0? and what is the variable here finally? $0 is
> what it interprets? if yes so it is not a legal variable
> I am totally mixed up.

The variable ${0} is a variable variable. For instance:

     $foo = 50;
     $fee = 'foo';
     echo ${$fee}."\n";   // This will output 50.
     echo ${'foo'}."\n";  // Again this will out 50.

To leave off the closing curly brace is to leave the expression 
unterminated. Similar to leaving off a parenthesis, square brace, or 
conditional brace. The engine needs to know the end of the variable to 
which it should retrieve it's value to then find the actual variable to 
look up in the current context. It is most like having an array and 
saying you want some indexed value from the array:

     $array[50]

You can't very well leave off the closing square brace or else the 
parser doesn't know where the index ends. In other words ${0} isn't a 
variable called "${0}" it's a variable referenced by the result of 
evaluating what lies between the curly braces. The following is 
perfectly valid:

     $foo1 = 1;
     $foo2 = 2;
     $foo3 = 3;

     for( $i = 1; $i <= 3; $i++ )
     {
         echo ${'foo'.$i};
         echo ${"foo$i"};   // Alternative.
     }

While you can't explicitly reference a variable called $0 you can set 
one into the context by using curly braces:

     ${0} = 100;
     echo ${0}."\n";

Since variables are denoted by the lead $ character I can't think of a 
good reason why a variable can't start with a number after the $ since 
the parser already knows it's working with a variable. I would imagine 
that it's a convention carried forward from C or other languages.

Cheers,
Rob.
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