PHP if not statements

Question

This may be the way my server is set up  but I m banging my head against the wall  I m trying to say that if  action has no value or has a value that is not  add  or  delete  then have an error  else keep running the script  However  I get an error no matter what  action is     action     GET  a     if   isset  action        action     add      action     delete            header  location index php         echo  error  lt br gt          action is being set properly and if run something like if  action    add   it works  This is on my local host  so it could be a settings issue

User · Answer

You re saying  if it s not set or it s different from add or it s different from delete   You realize that a    x  amp  amp  a    y  with x    y is necessarily false since a cannot be simultaneously two different values

User · Answer

I think this is the best and easiest way to do it   if    isset  action   amp  amp    action     add      action     delete

User · Answer

For future reference  you can quickly create a truth table to check if it evaluates the way you want     it s kind of like Sudoku     isset  action    amp  amp    action     add   amp  amp   action     delete      Example    column 1 is issetaction  column 2 and 3 evaluates    add   delete  respectively  if  a add  T  amp  amp   F  amp  amp  T     T  amp  amp  F    FALSE  if  a delete  T  amp  amp   T  amp  amp  F     T  amp  amp  F    FALSE  if  a nothing  T  amp  amp   T  amp  amp  T     T  amp  amp  T    TRUE

User · Answer

Your logic is slightly off  The second    should be  amp  amp    if    isset  action        action     add   amp  amp   action     delete      You can see why your original line fails by trying out a sample value  Let s say  action is  delete   Here s how the condition reduces down step by step       action     delete  if    isset  action        action     add      action     delete    if    true       action     add      action     delete    if  false      action     add      action     delete    if   action     add      action     delete   if  true     action     delete   if  true    false  if  true    Oops  The condition just succeeded and printed  error   but it was supposed to fail  In fact  if you think about it  no matter what the value of  action is  one of the two    tests will return true  Switch the    to  amp  amp  and then the second to last line becomes if  true  amp  amp  false   which properly reduces to if  false    There is a way to use    and have the test work  by the way  You have to negate everything else using De Morgan s law  i e    if    isset  action         action     add      action     delete      You can read that in English as  if action is not  either add or remove   then

User · Answer

Not an answer  but just for the sake of code formatting  if  isset   GET  a      action   GET  a    else  action      if    action      add  OR  action      delete       header  location   index php      exit      Note the exit  statement after header    That s the important thing  header   does not terminate script execution

User · Answer

No matter what  action is  it will always either not be  add  OR not be  delete   which is why the if condition always passes  What you want is to use  amp  amp  instead of        isset  action        action    add   amp  amp   action    delete

User · Answer

You could also try   if    isset  action         action     add      action     delete           Do your stuff

[php] PHP if not statements

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