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# Using Arrays

Arrays crop up in almost every PHP program. In addition to their obvious use for storing collections of values, they’re also used to implement various abstract data types. In this section, we show how to use arrays to implement sets and stacks.

## Sets

Arrays let you implement the basic operations of set theory: union, intersection, and difference. Each set is represented by an array, and various PHP functions implement the set operations. The values in the set are the values in the array—the keys are not used, but they are generally preserved by the operations.

The union of two sets is all the elements from both sets with duplicates removed. The `array_merge()` and `array_unique()` functions let you calculate the union. Here’s how to find the union of two arrays:

````function` `arrayUnion``(``\$a``,` `\$b``)`
`{`
`\$union` `=` `array_merge``(``\$a``,` `\$b``);` `// duplicates may still exist`
`\$union` `=` `array_unique``(``\$union``);`

`return` `\$union``;`
`}`

`\$first` `=` `array``(``1``,` `"two"``,` `3``);`
`\$second` `=` `array``(``"two"``,` `"three"``,` `"four"``);`

`\$union` `=` `arrayUnion``(``\$first``,` `\$second``);`
`print_r``(``\$union``);`

`Array``(`
`[``0``]` `=>` `1`
`[``1``]` `=>` `two`
`[``2``]` `=>` `3`
`[``4``]` `=>` `three`
`[``5``]` `=>` `four`
`)````

The intersection of two sets is the set of elements they have in common. PHP’s built-in `array_intersect()` function takes any number of arrays as arguments and returns an array of those values that exist in each. If multiple keys have the same value, the first key with that value is preserved.

## Stacks

Although not as common in PHP programs as in other programs, one fairly common data type is the last-in ...

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