array_flip() returns an array in flip order, i.e. keys from trans become values and trans's values become keys.
Note that the values of trans 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 flipped.
If a value has several occurrences, the latest key will be used as its values, and all others will be lost.
array_flip() returns FALSE if it fails.