如何通过PHP将DB上的值传递给JAVASCRIPT

Question

<?php
    // Connect to database server
    mysql_connect("192.168.1.101/phpmyadmin/", "root", "praticas") or die (mysql_error ());

    // Select database
    mysql_select_db("logs_history") or die(mysql_error());

    //---> at this point, I want yo pull a VALUE from a column called potencia_ativa and insert in 
    // the code JS below


      mysql_query($strSQL);
    // Close the database connection
    mysql_close();
    ?>

//JS code-------------------------------------------------------------------------------------------

<div id="graphHolder" style="height:75%;width:100%"></div>
<script type="text/javascript">

setInterval(
            function() {

                var d1 = [];
                for (var i = 0; i < parseInt(document.getElementById('number').value,10); i += 0.5) {
                    d1.push([  //----------> pull the value from the php/DB as a FLOAT
                        ]);
                }

                $.plot("#graphHolder", [{
                        label: "Label",
            data: d1,
                        lines: { show: true, fill: true }
                }
                ]);
            }, 1000);
</script>

Answer 1

Remember, PHP executes on the server and GENERATES the page that runs the Javascript on the client. Putting a value from PHP into the JS code is as simple as:

<script>
   var foo = <?php echo json_encode('bar'); ?>;
</script>

If you need to send the PHP data over AFTER the page was generated and already send to the client, then you need to have the client execute an AJAX request to fetch that data from the server. Once PHP has shoved the page out the door, it's basically done and can't "reach out" to the client to do updates on its own.

Also: Note the use of json_encode(). You should NEVER dump output from PHP directly into a Javascript context without it. Anything else puts you at risk of generating JS syntax errors, which will kill the entire JS code block.