Point of extra space when using strpos - php

Question

I have seen this in several functions that use strpos, but can't wrap my head around the purpose of adding an extra space. Can someone please help me understand this.
Example is the function get_string_between (see below) which returns whatever is between 2 tags. So for the string "this is my [tag]dog[/tag]" it will return-> dog
I understand what the function does, but what is the purpose of
$string = ' ' . $string;
Inside the following:
function get_string_between($string, $start, $end){
$string = ' ' . $string;
$ini = strpos($string, $start);
if ($ini == 0) return '';
$ini += strlen($start);
$len = strpos($string, $end, $ini) - $ini;
return substr($string, $ini, $len);
}
$fullstring = 'this is my [tag]dog[/tag]';
$parsed = get_string_between($fullstring, '[tag]', '[/tag]');
echo $parsed; // (result = dog)

Answer 1

Looks like they added it so they don't have to deal with when strpos returns false. It is a little odd way of doing it, but the result is the same.
Here is a more robust example:
function get_string_between($string, $start, $end) {
// String is empty, will never find anything
if (strlen($string) === 0) {
return '';
}
// Empty tags, not possible to find in the string
if (strlen($start) === 0 || strlen($end) === 0) {
return '';
}
$startIndex = strpos($string, $start);
if ($startIndex === false) {
return ''; // Start not found in string
}
// Get end with offset of start, so it must appear
// after the start tag
$endIndex = strpos($string, $end, $startIndex);
if ($endIndex === false) {
return ''; // End not found in string
}
$from = ($startIndex + strlen($start));
$length = ($endIndex - $from);
return substr($string, $from, $length);
}
Or you can use a regular expression:
function get_string_between($string, $start, $end) {
if (strlen($string) === 0 || strlen($start) === 0 || strlen($end) === 0) {
return '';
}
$start = preg_quote($start, '/');
$end = preg_quote($end, '/');
preg_match("/(?<=$start).*?(?=$end)/", $string, $result);
if (count($result) > 0) {
return array_shift($result);
}
return '';
}