title: PHP arithmetic produces exponent when should produce 0
body: <p>I have application that is adding a series of numbers.  I expect the numbers to sum to equal <code>0</code>, but when PHP processes the math, it comes out to <code>-1.2789769243682E-13</code>.  I am curious why this is.  When I perform the math manually it equals zero like expected.</p>
  <pre><code>&lt;?php
  
  $math = 138.57 + 138.57 + 138.57 + 138.57 + 138.57 + (-673.71) + (-19.14);
  echo $math;
  //-1.2789769243682E-13
  </code></pre>