In PHP, the explode() and implode() functions are used for string manipulation.

1. explode() function:

The explode() function is used to split a string into an array based on a specified delimiter. The syntax for explode() function is:

$array = explode($delimiter, $string);

Where $delimiter is the character or substring that you want to use as a delimiter to split the string and $string is the string that you want to split.

For example, if you have a string Hello World, and you want to split it into an array based on the space character, you can use the following code:

$string = "Hello World";
$array = explode(" ", $string);
print_r($array);

This will output the following array:

Array
(
    [0] => Hello
    [1] => World
)

2. implode() function:

The implode() function is used to join the elements of an array into a string using a specified separator. The syntax for implode() function is:

$string = implode($separator, $array);

Where $separator is the character or substring that you want to use as a separator to join the elements of the array and $array is the array that you want to join.

For example, if you have an array ["Hello", "World"], and you want to join it into a string using a space character as a separator, you can use the following code:

$array = array("Hello", "World");
$string = implode(" ", $array);
echo $string;

Hello World

3. Limiting the number of elements returned by explode():

The explode() function also allows you to limit the number of elements returned by passing a third parameter, which specifies the maximum number of elements to return. For example:

$string = "one,two,three,four,five";
$array = explode(",", $string, 3);
print_r($array);

This will output the following array:

Array
(
    [0] => one
    [1] => two
    [2] => three,four,five
)

In this example, we have limited the explode() function to return only three elements, so the last three elements of the string are returned as a single element in the array.

4. Using implode() to join array elements with a different separator:

By default, implode() joins the array elements using a space character as a separator. However, you can specify any separator you want by passing it as the first argument to the function. For example:

$array = array("one", "two", "three");
$string = implode("-", $array);
echo $string;

This will output the following string:

one-two-three

In this example, we have used a dash (-) character as a separator instead of a space character.