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>Variable variables</H1
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>&#13;    Sometimes it is convenient to be able to have variable variable
    names.  That is, a variable name which can be set and used
    dynamically.  A normal variable is set with a statement such as:
   </P
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>&#60;?php
$a = "hello";
?&#62;</PRE
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>&#13;    A variable variable takes the value of a variable and treats that
    as the name of a variable.  In the above example,
    <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>hello</I
></SPAN
>, can be used as the name of a variable
    by using two dollar signs. i.e.
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>&#60;?php
$$a = "world";
?&#62;</PRE
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>&#13;    At this point two variables have been defined and stored in the
    PHP symbol tree: <TT
CLASS="varname"
>$a</TT
> with contents "hello" and
    <TT
CLASS="varname"
>$hello</TT
> with contents "world".  Therefore, this
    statement:
   </P
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>&#60;?php
echo "$a ${$a}";
?&#62;</PRE
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>&#13;    produces the exact same output as:
   </P
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>&#13;    i.e. they both produce: <TT
CLASS="computeroutput"
>hello world</TT
>.
   </P
><P
>&#13;    In order to use variable variables with arrays, you have to
    resolve an ambiguity problem.  That is, if you write
    <TT
CLASS="varname"
>$$a[1]</TT
> then the parser needs to know if you
    meant to use <TT
CLASS="varname"
>$a[1]</TT
> as a variable, or if you
    wanted <TT
CLASS="varname"
>$$a</TT
> as the variable and then the [1]
    index from that variable.  The syntax for resolving this ambiguity
    is: <TT
CLASS="varname"
>${$a[1]}</TT
> for the first case and
    <TT
CLASS="varname"
>${$a}[1]</TT
> for the second.
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>&#13;     Please note that variable variables cannot be used with PHP's 
     <A
HREF="language.variables.predefined.html#language.variables.superglobals"
>Superglobal arrays</A
>.
     This means you cannot do things like <TT
CLASS="varname"
>${$_GET}</TT
>. If you are 
     looking for a way to handle availability of superglobals and the old
     <TT
CLASS="varname"
>HTTP_*_VARS</TT
>, you might want to try 
     <A
HREF="language.references.html"
>referencing</A
> them.
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