array_flip returns an array in flip order, i.e. keys from array become values and values from array become keys.
Note that the values of array need to be valid keys, i.e. they need to be either int 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.
An array of key/value pairs to be flipped.
Returns the flipped array.