To merge an array of strings into a single string split by a new line in PHP, you can use the `implode` function to join the array elements into a single string, with the new line character (`\n`) as the delimiter.

Here's an example code snippet:

// An array of strings
$array_of_strings = array('Hello', 'world', 'in', 'PHP');

// Merge the array into a single string with new line delimiter
$merged_string = implode("\n", $array_of_strings);

// Output the merged string
echo $merged_string;

Output:

Hello
world
in
PHP

In the above code, the `implode` function takes two arguments - the delimiter and the array to be merged. The delimiter here is the new line character (`\n`) which separates each element of the array in the resulting string.

You can modify the delimiter to any other character or string as per your requirement.