<?php
      $show_value   = 123;
      echo 'sing_quote'.$show_value;
      echo "double_quote{$show_value}";

  ?>

Его код операции:

1: <?php
2: $show_value   = 123;
        0  ASSIGN              !0, 123
3: echo 'sing_quote'.$show_value;
        1  CONCAT              'sing_quote', !0 =>RES[~1]     
        2  ECHO                ~1
4: echo "double_quote{$show_value}";
        3  ADD_STRING          'double_quote' =>RES[~2]     
        4  ADD_VAR             ~2, !0 =>RES[~2]     
        5  ECHO                ~2
        6  RETURN              1