What is the difference between using print_r and echo when outputting array elements in PHP?

When outputting array elements in PHP, the main difference between using print_r and echo is that print_r is specifically designed for displaying the contents of arrays and objects in a human-readable format, while echo is a more general-purpose function for outputting strings. Print_r will display the entire array structure, including keys and values, while echo will simply output the value of a single array element at a time. Therefore, if you want to quickly and easily view the contents of an array, print_r is the preferred choice.

```php
// Using print_r to output array elements
$array = array("apple", "banana", "cherry");
print_r($array);
```
In this example, the print_r function will display the entire array structure, including all elements and their respective keys.