Use variable from template.php in block
You need to use a preprocess function.  Take a look at template_preprocess_block().  In your template.php you can use:

function YOURTHEME_preprocess_block(&amp;$variables) {
  $variables['sub_count'] = db_query("SELECT count FROM {sub_count}")-&gt;fetchField();
}


and can print into the .tpl.php file as you did above.

A few good articles to learn more about preprocess functions are


About overriding themable output
Setting up variables for use in a template (preprocess and process functions)


As a basic overview of preprocess functions, from the second article:


  Preprocess functions only apply to theming hooks implemented as
  templates. The main role of the preprocessor is to set up variables to
  be placed within the template (.tpl.php) files. Plain theme functions
  do not interact with preprocessors.  
  
  Notes:
  
  
  Preprocessors are also used for providing template suggestions.  
  In versions 5 and below, the function _phptemplate_variables served the
  same purpose. It has been deprecated in 6.  
  Prior to Drupal 6.7, for your theme to have its preprocessors recognized, the template
  associated with the hook had to exist inside the theme. When a default
  template exists, copy it to your theme and clear the registry (or you
  should really be upgrading to a later version of Drupal anyway for
  security reasons, at which point you don't have to worry about this).