create_function() and escape\ character question

[TheKeith]

Hi, I'm just learning php now for the first time and I'm having a little
trouble understanding something.

In the following example:

------------------------------------------------------------------------

<?php

function test ($thenum) {
$funct = create_function ("\$thenumbe r", "return (\$thenumber+5) ;");
echo $funct($thenum) ;
}

test(5);

?>

------------------------------------------------------------------------

I don't understand why the escape characters are necessary here. I
understand that the create_function () function requires its parameters
to be in string format, but if my $thenum variable is an integer and not
a string, why is the \ necessary?

Furthermore, why does this next single quote version work without the \:

------------------------------------------------------------------------

<?php

function test ($thenum) {
$funct = create_function ('$thenumber', 'return ($thenumber+5); ');
echo $funct($thenum) ;
}

test(5);

?>

------------------------------------------------------------------------

Is this behavior something I will encounter often in php, because I
never had this confusion doing javascript or actionscript?


Help would be appreciated.

Keith

[Alvaro G Vicario]

Re: create_function () and escape\ character question

*** TheKeith wrote/escribió (Mon, 07 Jun 2004 19:41:55 -0400):[color=blue]
> I don't understand why the escape characters are necessary here. I
> understand that the create_function () function requires its parameters
> to be in string format, but if my $thenum variable is an integer and not
> a string, why is the \ necessary?
>
> Furthermore, why does this next single quote version work without the \:[/color]

Sorry, I'm not advanced enough to understand the purpose of
create_function () but I can tell you the difference between single and
double quotes:

* Single quotes are treated as literals:

$foo=33;
echo 'This is $foo'; // prints: This is $foo

* Double quotes are parsed and variables and special chars get replaced by
their values

$foo=33;
echo "This is $foo"; // prints: This is 33

--
--
-- Álvaro G. Vicario - Burgos, Spain
--

[Andy Hassall]

Re: create_function () and escape\ character question

On Mon, 07 Jun 2004 19:41:55 -0400, TheKeith <no@spam.com> wrote:
[color=blue]
><?php
>function test ($thenum) {
> $funct = create_function ("\$thenumbe r", "return (\$thenumber+5) ;");
> echo $funct($thenum) ;
>}
>test(5);
>?>
>
>------------------------------------------------------------------------
>
>I don't understand why the escape characters are necessary here. I
>understand that the create_function () function requires its parameters
>to be in string format, but if my $thenum variable is an integer and not
>a string, why is the \ necessary?[/color]

Because otherwise $thenumber inside the function definition gets interpolated
to a value _before_ it reaches create_function .

Consider without the escapes:

function test ($thenum) {
$funct = create_function ("$thenumber ", "return ($thenumber+5); ");
echo $funct($thenum) ;
}

Forget that this is a create_function for the moment, and just look at the
strings.

"$thenumber "

is the same as

$thenumber

i.e. undefined, as there's no variable of that name in scope at the moment.

"return ($thenumber+5); "

is similarly

"return (+5);"

... because there's no $thenumber in this scope.

But with the escapes in, the text gets passed to create_function as is, and so
$thenumber is a variable _within the scope of the newly created function_.
[color=blue]
>Furthermore, why does this next single quote version work without the \:
>
>------------------------------------------------------------------------
>
><?php
>function test ($thenum) {
> $funct = create_function ('$thenumber', 'return ($thenumber+5); ');
> echo $funct($thenum) ;
>}
>test(5);
>?>[/color]

Similar reason - except here, single quotes don't interpolate variables
anyway, so you don't need to escape $.
[color=blue]
>Is this behavior something I will encounter often in php, because I
>never had this confusion doing javascript or actionscript?[/color]

Double quotes (which interpolate any variables inside) versus single quotes
(which don't) are all over the place in PHP. The confusion here is more due to
the interaction with create_function - where you don't want the variables
interpolated now, you want it to happen when the anonymous function executes.

--
Andy Hassall <andy@andyh.co. uk> / Space: disk usage analysis tool
http://www.andyh.co.uk / http://www.andyhsoftware.co.uk/space

[TheKeith]

Re: create_function () and escape\ character question

Andy Hassall wrote:
[color=blue]
> On Mon, 07 Jun 2004 19:41:55 -0400, TheKeith <no@spam.com> wrote:
>
>[color=green]
>><?php
>>function test ($thenum) {
>> $funct = create_function ("\$thenumbe r", "return (\$thenumber+5) ;");
>> echo $funct($thenum) ;
>>}
>>test(5);
>>?>
>>
>>------------------------------------------------------------------------
>>
>>I don't understand why the escape characters are necessary here. I
>>understand that the create_function () function requires its parameters
>>to be in string format, but if my $thenum variable is an integer and not
>>a string, why is the \ necessary?[/color]
>
>
> Because otherwise $thenumber inside the function definition gets interpolated
> to a value _before_ it reaches create_function .
>
> Consider without the escapes:
>
> function test ($thenum) {
> $funct = create_function ("$thenumber ", "return ($thenumber+5); ");
> echo $funct($thenum) ;
> }
>
> Forget that this is a create_function for the moment, and just look at the
> strings.
>
> "$thenumber "
>
> is the same as
>
> $thenumber
>
> i.e. undefined, as there's no variable of that name in scope at the moment.
>
> "return ($thenumber+5); "
>
> is similarly
>
> "return (+5);"
>
> ... because there's no $thenumber in this scope.
>
> But with the escapes in, the text gets passed to create_function as is, and so
> $thenumber is a variable _within the scope of the newly created function_.
>
>[color=green]
>>Furthermore , why does this next single quote version work without the \:
>>
>>------------------------------------------------------------------------
>>
>><?php
>>function test ($thenum) {
>> $funct = create_function ('$thenumber', 'return ($thenumber+5); ');
>> echo $funct($thenum) ;
>>}
>>test(5);
>>?>[/color]
>
>
> Similar reason - except here, single quotes don't interpolate variables
> anyway, so you don't need to escape $.
>
>[color=green]
>>Is this behavior something I will encounter often in php, because I
>>never had this confusion doing javascript or actionscript?[/color]
>
>
> Double quotes (which interpolate any variables inside) versus single quotes
> (which don't) are all over the place in PHP. The confusion here is more due to
> the interaction with create_function - where you don't want the variables
> interpolated now, you want it to happen when the anonymous function executes.
>
> --
> Andy Hassall <andy@andyh.co. uk> / Space: disk usage analysis tool
> http://www.andyh.co.uk / http://www.andyhsoftware.co.uk/space[/color]


Ahh thanks, I think I understand now--so if it were put with the double
quotes but without the escape, it would be the same as:

------------------------------------------------------------

<?php

function test ($thenum) {
$funct = create_function ("", "return (""+5);");
echo $funct($thenum) ;
}

test(5);

?>

-------------------------------------------------------------

since $thenumber has no value associated with it at the time of the
create_function () function's initial execution, correct? In this case,
my confusion was due to the fact that I assumed the create_function ()
function worked like a regular function in not reading its contents
until specifically called, but that's not the way it works at all, right?

Thanks again.