Chapter 5. Variables
Along with conditional logic, variables are the core of what makes computer programs powerful and flexible. If you think of a variable as a bucket with a name that holds a value, PHP lets you have plain old buckets, buckets that contain the name of other buckets, buckets with numbers or strings in them, buckets holding arrays of other buckets, buckets full of objects, and just about any other variation on that analogy you can think of.
A variable is either set or unset. A variable with any value assigned to it,
false, empty or nonempty, is set. The function
true when passed a variable that’s set. To turn a variable that’s set into one that’s unset, call
unset() on the variable or assign
null to the variable. Scalars, arrays, and objects can all be passed to
unset(). You can also pass
unset() multiple variables to unset them all:
If a variable is present in the query string of a URL, even if it has no value assigned to it, it is set in the appropriate superglobal array. Thus:
$_GET['chimps'] to the empty string.
All unset variables are also empty. Set variables may be empty or nonempty. Empty variables have values that evaluate to
false as a boolean. These are listed in Table 5-1.
“” (empty string)