Explain the actions of the implode and explode functions.

The `implode()` function in PHP is used to join elements of an array into a string with a specified separator. It takes two parameters: the first parameter is the separator that will be used to join the array elements, and the second parameter is the array of elements to be joined. The function returns a string that is formed by joining the elements of the array with the specified separator.

For example, if we have an array of strings like this:

$arr = array('apple', 'banana', 'pear');

We can join the elements of the array into a string using the `implode()` function like this:

$str = implode(',', $arr);

This will create a string like this:

"apple,banana,pear"

The `explode()` function, on the other hand, is used to split a string into an array of substrings using a specified separator. It takes two parameters: the first parameter is the separator that will be used to split the string, and the second parameter is the string to be split. The function returns an array of substrings.

For example, if we have a string like this:

$str = "apple,banana,pear";

We can split the string into an array of substrings using the `explode()` function like this:

$arr = explode(',', $str);

This will create an array like this:

array('apple', 'banana', 'pear')

So, basically, the `implode()` function joins an array of elements into a string using a specified separator, while the `explode()` function splits a string into an array of substrings using a specified separator.