/**
 * 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('', $str2);
}
//范例演示：
print MergeBetween('abcdef', '__') . "\n";
print MergeBetween('__', 'abcdef') . "\n";
print MergeBetween('bb', 'aa') . "\n";
print MergeBetween('aa', 'bb') . "\n";
print MergeBetween('a', 'b') . "\n";
/*
Output:
a_b_cdef
a_b_cdef
baba
abab
ab
*/