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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.

Get javascript variable value in php

I need to get javascript variable value in php file.
html example:
UPDATE:
$html = '
<script>
window.runParams.adminSeq="3423423423423";
window.runParams.companyId="2349093284234";
</script>';
Shout I use regex ? regex is very complex to me... any help ?

<?php
$html = '<script>
window.runParams.adminSeq="3423423423423";
window.runParams.companyId="2349093284234";
</script>';
$variables = ["adminSeq", "companyId"];
$counter = 0;
foreach($variables as $variable) {
preg_match_all('/"(.*?)"/', $html, $matches);
${"$variable"} = ($matches[1])[$counter];
$counter++;
}
echo $adminSeq; // Prints out: 3423423423423
echo $companyId; // Prints out: 2349093284234
?>
You can also use GET requests to do this. The link would look like http://localhost/?adminSeq=3423423423423&companyId=2349093284234 then get out these values in PHP with:
<?php
$adminSeq = $_GET["adminSeq"];
$companyId = $_GET["companyId"];
?>

How to retrieve $_SESSION value in a JavaScript/HTML

I have this working script yet when I change it to retrieve(supposedly) the value inside $_SESSION["username"], it doesn't retrieve anything. The whole page is saved as .php as I am using some codes that uses PHP.
Code:
echo "<script type=text/javascript>";
echo "var hidden = false;";
echo "function actiondb1() {";
echo "if(!hidden) {";
echo "document.getElementById(\'clickdb1\').style.visibility = \'hidden\';";
echo "document.getElementById(\'db1\').style.visibility = \'visible\';";
echo "document.getElementById(\'db1\').disabled = false;";
echo "document.getElementById(\'db1\').value =".$_SESSION["username"];.";";
echo "}";
echo "}";
echo "</script>";
How can I make the script to properly retrieve the data inside $_SESSION["username"];?

Observe that, for instance, if the value of $_SESSION["username"] is John, your echo will be generating this result:
document.getElementById('db1').value = John;
But John is supposed to be a string and should be wrapped in quotation marks, otherwise JavaScript will understand it as a variable name (which value will be probably undefined).

As Havenard mentioned, this line is missing Javascript single quotes to properly denote a string variable:
echo "document.getElementById(\'db1\').value ='".$_SESSION["username"];."';";
However, you really shouldn't print JS out with PHP if you can help it. Though iatboy's answer answer won't ultimately fix your bug, it is a much cleaner way of doing things.
?>
<script type=text/javascript>;
var hidden = false;
function actiondb1() {
if(!hidden) {
document.getElementById('clickdb1').style.visibility = 'hidden';
document.getElementById('db1').style.visibility = 'visible';
document.getElementById('db1').disabled = false;
document.getElementById('db1').value ='<?php echo $_SESSION["username"];?>';
}
}
</script>;
<?php

Did you start session in this page?If you didn't,use the follow code to start session.
session_start();
Then change your code to
echo "<script type=text/javascript>";
echo "var hidden = false;\n";
echo "function actiondb1() {\n";
echo "alert('".$_SESSION['username']."')\n"; //test code
echo "if(!hidden) {\n";
echo "document.getElementById('clickdb1').style.visibility = 'hidden';\n";
echo "document.getElementById('db1').style.visibility = 'visible';\n";
echo "document.getElementById('db1').disabled = false;\n";
echo "document.getElementById('db1').value ='".$_SESSION["username"]."';\n";
echo "}\n";
echo "}\n";
echo "</script>";
it will be ok.

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>

.replace() not working inside a jQuery function

I don't understand why replace() function doesn't work into my jQuery function:
jQuery(document).ready(function($){
var amount_min = <?php if($_GET['amount_min']) echo $_GET['amount_min']; else echo '0'; ?>;
var amount_min = amount_min.replace(/[^\d]/g, "");
$('input[name=amount]').val(amount_min);
});
Whatever input I give (for example "100ab" or "10.000") it doesn't replace it with "100" or "10000".
How to do?

You forgot to put double-quotes.
var amount_min = "<?php if($_GET['amount_min']) echo $_GET['amount_min']; else echo 0; ?>";
Because, replace works in Strings.
UPDATE #1
If for any religious reason you don't want to wrap the PHP in double quotes then output them along with the number.
var amount_min = <?php echo '"' . ($_GET['amount_min'] ? $_GET['amount_min'] : 0) . '"'; ?>;
UPDATE #2
Compulsory validation you can use:
var amount_min = <?php echo '"' . (int)($_GET['amount_min']) . '"'; ?>;

can you please try this:
$(document).ready(function(){
var amount_min = "<?php if($_GET['amount_min']){ echo $_GET['amount_min'];}else{ echo '0';} ?>";
console.log("original-> "+amount_min);
var amount_min = amount_min.replace(/\D/g,'');
console.log("replaced-> "+amount_min);
});

Your PHP code is outputting a number:
var amount_min = 100;
Since you're expecting a string, wrap it in quotes:
var amount_min = "<?php if($_GET['amount_min']) echo $_GET['amount_min']; else echo '0'; ?>";
I haven't touched PHP in years, but I think you could simplify your code a little:
var amount_min = "<?php echo($_GET['amount_min'] || '0'); ?>";
Also, why don't you just fetch the GET parameter with JavaScript?

You don't need the .replace() code (it also only works on strings); PHP can already do the proper conversion for you:
$(function() {
var amount_min = <?php echo isset($_GET['amount_min']) ? (int)$_GET['amount_min'] : 0; ?>;
$('input[name=amount]').val(amount_min);
});
You could also use filtering for this:
<?php echo filter_input(INPUT_GET, 'amount_min', FILTER_VALIDATE_INT, array('options' => array('default' => 0))); ?>
Note that filter_input would not accept a value like 100abc, so use wisely.
If you still want to use strings safely in JavaScript you should use json_encode().
Btw, any answer that involves an unmodified echo of a request variable from PHP inside JavaScript code is wrong and can cause XSS attacks! You have been warned.
Update
The regular expression based replacement can also be done in PHP:
var amount_min = <?php echo (int)preg_replace('/\D+/', '', isset($_GET['amount_min']) ? $_GET['amount_min'] : 0); ?>;
Since all non-digits are removed, you can safely apply the (int) cast.