How to prevent simultaneous use of a form in a CMS with php and Mysql

[tonvandelangkru]

I have developed a CMS system with Php and Mysql. As more people are working on it simutanaeously, there is the danger that two users have the same html form on the screen, and one user changes fields, while the other is then working on old information. When this latter submits, he will destroy the changes that the first user made.
Is there a neat way to prevent this? Blocking certain pages, making a warning? How does that work in PHP and/or Mysql?

[kakki]

hmmm....

you'll need semaphores for this
( see php.net )

i solved the same problem by creating a file ( as the locking mechanism )
so ill have code like this

while ( file_exists( "semaphore.tmp" ) )
{

}
fclose ( fopen ( "semaphore.tmp", "w" ) ); // to create a 0 bytes file
// do sth restricted
unlink ( "semaphore.tmp" ); // to delete the 0 byte file

or you can use the sem_aquire, sem_get ( whatever ) mechanism of php
( if its compiled in )

[tonvandelangkru]

Do you mean that for every page that has a form in it, I should make php create the temporary file, and that that file will be automatically deleted once the user has left this page, so that someone else can open it?
And, if so does php recognize the complete name of the page, so for instance testform.php?id=4&name=john

If not, it would mean that e.g. working simultaneoulsly on two different forms for the editing of and adresss in an adress database would be impossible.

Thanks,

Ton

[kakki]

yeah...

or you can create a sub-directory for every table, you want to prevent multiple access in
and set filenames( in your example ) like "4john.sem" ... if someone would want to edit
this row in the table your script checks for this concat-filename ....

[tonvandelangkru]

Do you know of an exemple, so that I can get a better idea how to do this?

[kakki]

( my idea on the sub-dirs )DRAFT:
table: customers
id:int, primary key
name:varchar,
email:varchar ........

you create one sub-dir named "customers"
every time one starts editing a row
( EXAMPLE:id=10, name="john doe", email="[e-mail]" )
-the script checks for a file
( filename: "customers/10.sem" )
-creates a file named as the content of the "id"-field

everytime the script does the "update" statements, the script delete the file

the only problem is with people, who just open an entry for editing and
then close the browser( and leave you inbetween a session )

REAL EXAMPLES ON THIS:
some are:
- reader-/writer-cooperation in operating systems like windows/linux/unix ....
( mr tanenbaum from the university of amsterdam for example )

maybe you can check out some of the university-stuff ( they call that kind of code
"semaphore" ) ... but i am from germany and so i havent any international
urls on that.

the php.net site seems to be a good start as well ( sem_get, sem_release ) ......