title: PDO valid characters for placeholders
body: <p>In PHP with PDO, what characters are we limited to using. I've tried looking in the documentation and online but to no avail.</p>
  
  <p>I did find a post where a user had used a hypen in the name which broke the query. I'm writing a function that dynamically generates these names and since hyphens are no nos, I was wondering if there was a list of alternatives.</p>
  
  <pre><code>&lt;?php
  /* Execute a prepared statement by binding PHP variables */
  $calories = 150;
  $colour = 'red';
  $sth = $dbh-&gt;prepare('SELECT name, colour, calories
      FROM fruit
      WHERE calories &lt; :calories AND colour = :colour');
  $sth-&gt;bindParam(':calories', $calories, PDO::PARAM_INT);
  $sth-&gt;bindParam(':colour', $colour, PDO::PARAM_STR, 12);
  $sth-&gt;execute();
  ?&gt;
  </code></pre>
  
  <p>So in this example what characters would be allowed in the string ':colour'?</p>