codeigniter - PHP Write string to new line for the first time and not at the end of file -

using codeigniter function write_file(), can write existing file this:

write_file("path_to_file", "string four"."\n", "a+"); 

assume have file this:

string 1 string 2 string 3 

with write_file("path_to_file", "string four"."\n", "a+") , have ouput below when writing file first time:

string 1 string 2 string 3 string 4 

any idea how can write string four new line first time? shown below:

string 1 string 2 string 3  string 4 

thanks.

not knowing write_file() function, given said, should work :

write_file("path_to_file", "\n"."string four"."\n", "a+")