<?php
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
  // Retrieve the submitted value
  $value = $_POST['value'];

  // Process the value (for this example, we'll just double it)
  $processed_value = $value * 2;

  // Generate the graph data
  $x_data = array(0, 1, 2);
  $y_data = array($processed_value, $processed_value+1, $processed_value+2);

  // Encode the data as JSON for use in the JavaScript below
  $graph_data = json_encode(array('x' => $x_data, 'y' => $y_data));
}
?>

<!-- HTML form -->
<form method="post">
  <label for="value">Enter a value:</label>
  <input type="text" name="value" id="value">
  <button type="submit">Submit</button>
</form>

<!-- JavaScript to generate the graph -->
<?php if (isset($graph_data)): ?>
  <div id="graph"></div>
  <script src="https://cdn.plot.ly/plotly-latest.min.js"></script>
  <script>
    var graph_data = <?php echo $graph_data ?>;
    var trace = {
      x: graph_data.x,
      y: graph_data.y,
      mode: 'lines+markers',
      type: 'scatter'
    };
    var data = [trace];
    var layout = {
      title: 'Graph of <?php echo $processed_value ?>',
      xaxis: {
        title: 'X-axis'
      },
      yaxis: {
        title: 'Y-axis'
      }
    };
    Plotly.newPlot('graph', data, layout);
  </script>
<?php endif; ?>