Limiting the process to run by 1 user only ?

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Hello,

I have coded a custom mod.

in that there is a process like..

PHP Code:

//its an auction system

if($_REQUEST['do']=='placebidprocess')

{
      // some process
      // here i also have put one condition that if presentbid>newbid then it prints error
      // later if all fine it updates the value of presentbid as the newbid amount

} 


but this doesnt help me when 2 ppl run this process at the same time..
so the problem is when 2 ppl do it as same time.. 2 identical bids are placed by 2 differnt users
how can i avoid such a problem ?

thank you