Logical Operators

AND   Are both values true? e.g $variable AND $variable2

if ($username =='user' AND $password =='password')

{
   echo "Welcome user";
}
else

{
   echo "Invalid Login Detected";
}

XOR   Is at least one value true,but NOT both? e.g $variable XOR $variable2

if ($username =='user' XOR $username =='user2')

{
   echo "Welcome ".$username;
}
else

{
   echo "Invalid Login Detected";
}

OR   Is at least one value true? e.g $variable OR $variable2

if ($username =='user' OR $username =='user2')

{
   echo "Welcome ".$username;
}
else

{
   echo "Invalid Login Detected";
}

!   Is NOT something e.g $variable

&&   Are both values true? e.g $variable && $variable2