array_flip() returns an array in flip
order, i.e. keys from
array become values and values
array become keys.
Note that the values of
array need to be valid
keys, i.e. they need to be either integer or
string. A warning
will be emitted if a value has the wrong
type, and the key/value pair in question will not be included
in the result.
If a value has several occurrences, the latest key will be used as its value, and all others will be lost.