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how to pass php variable to javascript?

I am using following code to pass PHP variables to javascript. but it is not working.
function gto(str) {
document.getElementById('goto').action = str;
document.getElementById('ID').value = <?php echo "$userid" ?>;
document.getElementById('name').value = <?php echo "$user_name" ?>;
document.getElementById('gname').value = <?php echo "$usergname" ?>;
document.getElementById('fmname').value = <?php echo "$userfname" ?>;
document.getElementById('img').value = <?php echo "$userimg" ?>;
document.getElementById('email').value = <?php echo "$useremail" ?>;
document.getElementById('goto').submit();
}
Following is the PHP code
<?php
if($_POST["name"] == null)
{
$user_name = 'Annomyous';
}
else{
$user_name = $_POST["name"];
$userid= $_POST["id"];
$usergname= $_POST["gname"];
$userfname= $_POST["fname"];
$userimg= $_POST["img"];
$useremail= $_POST["email"];
}
echo "<p style='color : white'>$user_name";
echo "$userid" ;
echo "$gname";
echo "$fname";
echo "$img";
echo "$email";
echo "$user_name";
echo "$user_name</p>";
$user_name =htmlspecialchars($user_name);
$user_name =str_replace("<script>","", $user_name);
?>
the output is a follows:
ReAlItY TuTs104598758504708047866ReAlItY TuTsReAlItY TuTs//this is php echo output.
JAVASCRIPT OUTPUT:-
function gto(str) {
document .getElementById('goto').action = str;
document.getElementById('ID').value = ;
document.getElementById('name').value = Annomyous;
document.getElementById('gname').value = ;
document.getElementById('fmname').value = ;
document.getElementById('img').value = ;
document.getElementById('email').value = ;
document.getElementById('goto').submit();
}
Function gto is called here:
<button class="w3-btn header-btn" onclick="gto('Contact.php');">Contact Us</button>
I can see in PHP output I am getting all variable output.
but nin juavascript im getting only Annonymous why????
I need to pass post variables to contact us so i am using the form tag and javascript but this is not working Please help me!
Thanks in Advance

values from php to js can be passed in many ways, here's one of them
try to pass the php values when calling the js function, like this:-
<button onclick="gto('Contact.php','<?php echo $userid;?>','<?php echo $username;?>');">Contact Us</button>
and then get values on js function like this:-
function gto(str,userid,username) {
document .getElementById('goto').action = str;
document.getElementById('ID').value = userid;
document.getElementById('name').value =username;
}
that's it, now use the values in js as your wish

Uh, found another issue. The js is in smart quotes, which won't work... "`" is invalid. use "'".
Also, w3schools can't handle php in their editor, so it's no use.
Example for POST requests: https://www.w3schools.com/code/tryit.asp?filename=FQSA8MJYGJ47
Hope this helps.

Pass PHP Variable To JavaScript And Use Alert To Display

I am attempting the below to pass the value in my php variable to JavaScript, but I never have the alert display on screen. What is the proper way to do such?
PHP
<?php $date = '20170101'; ?>
JavaScript
var datestring = <? php echo $date; ?>;
var year = datestring.substring(0,4);
alert(year);
Fiddle:
https://jsfiddle.net/3ed6opj3/
The desired result that I want to display in the alert is 2017 - the answers below to this point have all been showing <? p

You have to add " on between the php script
var datestring = "<?php echo $date; ?>";

You need to add quotes
<?php echo "\"".$date."\""; ?>;

Array value is replaced when second one stores in php

I am storing session values into array getting from URL and try to display it (getting size of product in a shopping cart).
But first value is replaced by second one.
My code:
if(isset($_POST['radio'])){
$_SESSION['sz']=$_POST['radio'];
$si=$_SESSION['sz'];
}
<a href="product_detail.php?pdt_id='.$pdid.'&add=' .$pdid .'&size='.$si.'"
class="cartBtn" onclick="return fun1()">Add to cart</a>';
?>
Display page:
$rt=$_GET['size'];
$_SESSION['wer']=$rt;
$array = $_SESSION['wer']; //Assigns session var to $array
//print_r($array);
echo $array[$x];
}

About First part mistakes :
<?php
if(isset($_POST['radio'])){
$_SESSION['sz']=$_POST['radio'];
$si=$_SESSION['sz'];
}
Add to cart';
?>
Your a tag is completely wrong as #Hearner said. It should be out of the php tag or inside with an "echo" like this :
echo "<a href='product_detail.php?pdt_id=".$pdid."&add=".$pdid."&size=".$si."' class='cartBtn' onclick='return fun1()'>Add to cart</a>";
You cannot access your $si variable out of your if statement. As written here over, if your $si isn't declared before (since you said that it was not your complete code...) then $si (in the link href) does not exist. You should therefore declare it before your if statement OR place your link (a tag) inside your if statement too!
What if your "$_POST['radio']" is NOT set ?? what happens? code missing... !! is $si declared anyway?
About Second part :
<?php
for( $x = 0, $max = count($array); $x < $max; ++$x ) {
$rt=$_GET['size'];
$_SESSION['wer']=$rt;
$array = $_SESSION['wer'];
echo $array[$x];
}
?>
I don't understand what you're trying to do here... need more code/information... cannot help more without your whole code...
EDIT :
Here is a very simple example to show you how to keep your get vars into a session array.
page one (pageOne.php):
<?php
session_start();
if(!(isset($_SESSION['myTest']))){
$_SESSION['myTest'] = "AWESOME";
$_SESSION['varToKeep'] = [];
}else{
echo "A session is already started. This is : ".$_SESSION['myTest']."<br/>";
if(count($_SESSION['varToKeep']>0)){
echo "There are ".count($_SESSION['varToKeep'])." vars in the array!<br/>";
for($i=0;$i<count($_SESSION['varToKeep']);$i++){
echo "Item ".$i." : ".$_SESSION['varToKeep'][$i]."<br/>";
}
}
}
echo "<br/>Click below to add a value in array<br/>";
$random = rand(1,100);
echo "<a href='pageTwo.php?mygetvar=STACKTEST".$random."'>Click here</a>";
?>
page two (pageTwo.php):
<?php
session_start();
echo "myTest value is : ".$_SESSION['myTest']."<br/><br/>";
$value = $_GET['mygetvar'];
$_SESSION['varToKeep'][] = $value;
echo "<a href='pageOne.php'>CLICK HERE TO RETURN ON PAGE ONE!</a>";
?>

how to initialize session inside function of a script tag?

<?php
// Start the session
session_start();
?>
<script src="../../js/jquery.js"></script>
<script>
function hello(y){
alert( y);
}
</script>
<?php
{
require 'db_connect.php';
$count=0;
$count = mysql_query ('SELECT COUNT(ques_no) FROM asg_cre_mcq') ;
$row = mysql_fetch_array($count);
//echo $count;
for ($y = 1; $y <=$row[0]; $y++)
{
echo "<button style='width:30px;height:30px' onclick='hello($y);'>". $y."</button>";
// $_SESSION["buttons"] =$y;
}
}
?>
I am dynamically generating buttons depending on the value coming from database. Then using onclick function I am performing check of click on button i.e. which button is clicked. what I want is to pass this vale of y i.e. button number to another php page in hello function using sessions. How can i do that?

This is not how $_SESSION is used. Right now, you are simply overwriting $_SESSION['button'] in each iteration of your for loop, in the end it will have the value of the last $y, e.g 2332.
To do what you want to achieve one wouldn't use $_SESSION, but, for example pass the value as a $_GET parameter and fetch that on the next page. To do so, you could for example fetch and pass $y within the "hello" function.
function hello(y){
window.location.href = 'theUrlYouWantToCall?buttonnumber=' + y
}
and GET this on the next page with
echo $_GET['buttonnumber'];

Setting JS variable to PHP variable error

I want to use a php login name in my javascript game...
I have this at the top of my php page that runs the javascript:
<?php
$username = (isset($_GET['username']) ? ($_GET['username']) : "dude");
?>
In my javascript I have this... which doesn't alert the name at all and throws a Uncaught ReferenceError: daniel is not defined error:
var name = <?php echo $username; ?>;
alert(name);
And then I have this... which displays the name correctly at the top of the HTML:
<?php
echo "Welcome back " . $username . "...";
?>
If the name is being displayed correctly server side when the page has loaded, why can't it be alerted out from the Javascript?
Thanks

You aren't delimiting the value as a JavaScript string. It may be a string on the server, but not on the client. Do the following:
Change this:
var name = <?php echo $username; ?>;
To this:
var name = "<?php echo $username; ?>";

Enclose your php code within quotes.Try like this :
<script type="text/javascript">
var name = '<?php echo $username; ?>';
alert(name);
</script>