How to Insert and Retrieve Empty Line into a string

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PHP

MySQL

1.
I want to Insert a blank / Empty Line into a string and then I have to add this string into the Mysql database
For example

$str1 = "This is the first string";
$str2 = "This is the second string";
$final_string = $str1 . <blank line> . $str2;

The output of this string must be like this

This is the first string

This is the second string 

What should I use between the both strings to insert a blank line
Note: (I don't want to use the html <br /> tag because it will be added to the database as it is.)

2.
I Have to convert the string on the base of the blank line
The string is such like

This is the first line
This is the second line
This is the third line
This is the forth line

This is the fifth line
This is the Sixth line
This is the seventh line

I have to convert this string to an array. The Array must have the first four lines (before the blank line) as its first element and the rest of the three lines (after the blank line) as its second element.

I want to use the explode() function to convert this string into an array
But MY Question is what should I use in explode() function as a separator because the separator in the string is blank line.