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Question about internal workings of references/variables.

php.internals

Robert Cummings

22 years ago

Hi Internals,

I'd like to confirm/refute some knowledge I think I have of the PHP
internal workings with respect to variables and references. I believe
that when a variable is assigned the value of another variable an
internal reference is created and some bit is used to keep track of
whether the value should be copied when an attempt is made to change
either the original variable or the newly assigned variable (maybe this
isn't the case for integers or floats since the performance would be
similar). Can anyone confirm or elaborate further?

Also I think I remember hearing once upon a time that creating a
reference in PHP script is more work than creating a copy when the copy
won't be altered? Can this be confirmed also?

Last but not least, do objects work under the same principles?

I did some benchmarking and my first question seems to hold since
assigning by value a 100000 entry array 10 million times runs as fast as
assigning a single integer 10 million times.

The second doesn't seem to hold true since the execution time is
slightly (but significantly and consistently) faster.

Thanks for any information.

Rob.

--
.------------------------------------------------------------.
| InterJinn Application Framework - http://www.interjinn.com |
:------------------------------------------------------------:
| An application and templating framework for PHP. Boasting  |
| a powerful, scalable system for accessing system services  |
| such as forms, properties, sessions, and caches. InterJinn |
| also provides an extremely flexible architecture for       |
| creating re-usable components quickly and easily.          |
`------------------------------------------------------------'

Derick Rethans

22 years ago

On Wed, 10 Mar 2004, Robert Cummings wrote:

> Hi Internals,
>
> I'd like to confirm/refute some knowledge I think I have of the PHP
> internal workings with respect to variables and references. I believe
> that when a variable is assigned the value of another variable an
> internal reference is created and some bit is used to keep track of
> whether the value should be copied when an attempt is made to change
> either the original variable or the newly assigned variable (maybe this
> isn't the case for integers or floats since the performance would be
> similar). Can anyone confirm or elaborate further?

yes, that is true and called "copy-on-write". It's done for all types of
variables though.

> Also I think I remember hearing once upon a time that creating a
> reference in PHP script is more work than creating a copy when the copy
> won't be altered? Can this be confirmed also?

Never tried that myself, but I believe it to be true.

>
> Last but not least, do objects work under the same principles?

yes, in php 4 they do. in php 5 it is true for "object-handles" and not
objects, but that shouldn't be of any concern.

regards,
Derick

Robert Cummings

22 years ago

On Wed, 2004-03-10 at 10:30, Derick Rethans wrote:

> On Wed, 10 Mar 2004, Robert Cummings wrote:
>
> > Hi Internals,
> >
> > I'd like to confirm/refute some knowledge I think I have of the PHP
> > internal workings with respect to variables and references. I believe
> > that when a variable is assigned the value of another variable an
> > internal reference is created and some bit is used to keep track of
> > whether the value should be copied when an attempt is made to change
> > either the original variable or the newly assigned variable (maybe this
> > isn't the case for integers or floats since the performance would be
> > similar). Can anyone confirm or elaborate further?
>
> yes, that is true and called "copy-on-write". It's done for all types of
> variables though.
>
> > Also I think I remember hearing once upon a time that creating a
> > reference in PHP script is more work than creating a copy when the copy
> > won't be altered? Can this be confirmed also?
>
> Never tried that myself, but I believe it to be true.
> >
> > Last but not least, do objects work under the same principles?
>
> yes, in php 4 they do. in php 5 it is true for "object-handles" and not
> objects, but that shouldn't be of any concern.

Thanks Derick, now I'll be sure to not misinform others :)

Cheers,
Rob

--
.------------------------------------------------------------.
| InterJinn Application Framework - http://www.interjinn.com |
:------------------------------------------------------------:
| An application and templating framework for PHP. Boasting  |
| a powerful, scalable system for accessing system services  |
| such as forms, properties, sessions, and caches. InterJinn |
| also provides an extremely flexible architecture for       |
| creating re-usable components quickly and easily.          |
`------------------------------------------------------------'