PHP methods for string function operations

/**
 * Merges two strings in a way that a pattern like ABABAB will be
 * the result.
 *
 * @param   string  $str1  String A
 * @param   string  $str2  String B
 * @return  string  Merged string
 */ 
function MergeBetween($str1, $str2){
  // Split both strings
  $str1 = str_split($str1, 1);
  $str2 = str_split($str2, 1);
  // Swap variables if string 1 is larger than string 2
  if (count($str1) >= count($str2))
    list($str1, $str2) = array($str2, $str1);
  // Append the shorter string to the longer string
  for($x=0; $x < count($str1); $x++)
    $str2[$x] .= $str1[$x];
  return implode(&#39;&#39;, $str2);
}
//范例演示：
print MergeBetween(&#39;abcdef&#39;, &#39;__&#39;) . "\n";
print MergeBetween(&#39;__&#39;, &#39;abcdef&#39;) . "\n";
print MergeBetween(&#39;bb&#39;, &#39;aa&#39;) . "\n";
print MergeBetween(&#39;aa&#39;, &#39;bb&#39;) . "\n";
print MergeBetween(&#39;a&#39;, &#39;b&#39;) . "\n";
/*
Output:
a_b_cdef
a_b_cdef
baba
abab
ab
*/