Question: do not understand ftell () solutions

Question: do not understand ftell () & lt ;? Php $ fp = fopen ('exa', 'r'); $ data = fgets ($ fp, 15); echoftell ($ fp); fclose ($ fp );? & Gt; output in this example: 13 if the value is changed to a relatively large number such as 1046, 13 is also output; if the value is changed to 13, 12 is output to ask new users: do not understand ftell ()
$ Fp = fopen ('exa', 'r ');

$ Data = fgets ($ fp, 15 );

Echo ftell ($ fp );

Fclose ($ fp );
?>

Output in this example: 13
If 15 is changed to a large number such as 1046, 13 is also output;
If 15 is changed to 13, 12 is output.


What is going on?

------ Solution --------------------
First, the pointer width of the row in your 'exa' file is 13, and the pointer starts from 0. Therefore, if you select 15 or larger, you only need to exceed the width of the row, only the maximum width, that is, 13, can be output.
Second, because the pointer is calculated from 0, $ data = fgets ($ fp, num); and the pointer actually starts from 0, then ftell ($ fp) the value must be a num-1, such as your fgets ($ fp, 5), then ftell ($ fp) will be 4.
Therefore, if you change 15 to 13, the output is 12. because 13 does not exceed the maximum width, the output is 13-1.