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how to call php function through hypertext link

I want to call a PHP function passing an argument on clicking a hyperlink (text with <a> tag) on the same page, i.e. href='#' onclick='loadpic($id).
Where $id is the variable to be passed to PHP function.

You can do that by ajax easily. Create a loadpic function in javascript instead and then pass $id to the php file via ajax and do something when you get the result.
Your javascript file should look like this-
(make sure you've inserted a link for jquery in your index file)
function loadpic(id_param){
$.post("filename.php",
{
id: id_param
},
function(data){
alert("Data received:" + data);
});
};
The data parameter in function(data) would be the value that you've echoed in the php file.

If you want to use onclick() method in your HTML element then you have to write your method is javascript and make an AJAX call to PHP. THis is one simple way to do this using php and javascript. And for this please see javascript and ajax with PHP.
But if you dont want to use javascript then here is a simple way and very simple code.
<?php
if(isset($_GET['myMethod'])){
myMethod($_GET['id']);
}
function myMethod($id){
echo $id."<br>";
}
?>
<?php
$myId = 4;
?>
<a href="index.php?id=<?php echo $myId ?>&myMethod" >MyLink</a>
Note: Copy this code and paste into a file called index.php because notice the hyper link tag and its attribute href="index.php".
Also note that you can not directly call php method using js like onclick() from your HTML tag. You should use AJAX or something. But using above demo code you can easily call myMethod() which is a PHP method using the hyper link tag.
In your case:
<?php
if(isset($_GET['call_loadpic'])){
loadpic($_GET['id']);
}
function loadpic($id){
echo "My loadpic method has been called! and the id is".$id."<br />";
}
?>
<!-- your other codes -->
<?php
$id = 4;
?>
<a href="this-same-file-name.php?id=<?php echo $id ?>&call_loadpic" >MyLink</a>
This is because your php function is in the same page and you want to call that function using same file and hyperlink in the same file. (According to your question)

You still will have to make javascript handle the exception because php has no onclick.
onClick=\"loadpic("'.$id.'");"\
//this will only send a php variable javascript will still have to loadpic();

wordpress - php and javascript dont go well together

I know some HTML and CSS but zero JAVA or PHP. I would like to insert LineIt button into my functions.php in WordPress:
The code goes like this:
<img src="imgpath" alt="LINE it!" />
Unfortunately this will display only the line link without my permalink. The_Permalink will be display as text not as dynamically taken URL.
I tried to do something like this:
<img src="linkin" width="[Width of Button]" height="[Height of Button]" alt="LINE it!" />'
But of course that didn't work. Oh and I don't want to use tags <script >.
Could help?
Thanks
Here is the function in my functions.php that I'm trying to modify with custom "LineIT" button:
function show() {
$return = '<aside class="show1 show2">'
. show_content() .
'<div class="show-box">'
. apply_filters('show_filter', show_rander()) .
'<div class="show3">'
. getIcons() .
'</div></div>**<img src="linkin" width="[Width of Button]" height="[Height of Button]" alt="LINE it!" />**<div style="clear:both;"></div>'
. show_content()
. show_content2() .
'</aside>
return apply_filters('show_buttons', $return);
}

according to the document at https://codex.wordpress.org/Function_Reference/the_permalink, the the_permalink() function is usually applied in the template within The Loop. Whereas functions.php is usually provide convenient functions and register them for the future use or register in The Loop.
So put the_permalink() in functions.php might not work as what you expected.
The general template files are like home.php, page.php or category-6.php(which will provide specific template for the category with id as 6).
In these files, the Loop usually is like:
<?php
if(have_posts()){
while(have_posts():
the_post();
the_content();
?>
//here you can add the link
<img src="imgpath" alt="LINE it!" />
<?php
endwhile;
endif;?>
Please refer to this page https://codex.wordpress.org/The_Loop_in_Action for further knowledge of the loop.
if you really want to use the functions.php, I would suggest using the other function get_permalink($id) and get_the_title($id) which will get the permalink of the post by the id and add a function for it. so the function will be:
function get_line($id){
$url = get_permalink($id);
$title = get_the_title($id);
return '<img src="imgpath" alt="LINE it!" />';
}
and then call the function in The Loop of the template.
To learn more about wordpress template, please refer https://developer.wordpress.org/themes/basics/template-hierarchy/

As suggested by #lhrec_106 I have used 3rd option and modified code works like a charm. Thanks!
I have inserted function get_line($id) before my function and then simply entered
. get_line($id) inside my custom function. Button works and share via Line my dynamically generated url.
Thank you

Storing the contents of an external php stylesheet in html

I have a php file that's being used as a dynamic stylesheet. So its a very large file with css/values generated by php. Here's an example.
<?php
header("Content-Type: text/css");
?>
.top {
top: <?= $topValue ?>px;
}
More css...
On a separate file, i'm trying to get the generated content from the php file and store it on the page somewhere so I can get it later with javascript. So the method i'm trying is storing the contents in a hidden input. Like so.
<?php
$file = file_get_contents("FILE_URL.php");
?>
<input type="hidden" name="hidden-css" value="<?= $file ?>" />
However, when I do this, the value contains the raw php from the file and its also commenting out the opening/closing php tags. So for instance, its doing things like '< !--?php' instead of "< ?php" and "?-->" instead of "?>". This messes up the general html on the page and doesn't store the value correctly in the input.
So I ask you, how can I fix the current situation or better store the contents of the stylesheet on the page so I can later grab it and use it with javascript?
EDIT: There may also be an issue with quotes in the generated css causing issues. I'm not sure how to address this issue either.
EDIT: This is how it should be stored if everything worked perfectly.
<input type="hidden" name="hidden-css" value="
.top {
top: 54px;
}
" />

You can't use the file_get_contents() function for this purpose - it simply gets the file contents (i.e. it won't run PHP on that file first).
Instead, as user574632 suggests, you must use include and so your code would look something like this:
<input type="hidden" name="hidden-css" value="<?php include 'FILE_URL.php'; ?>" />
Hopefully that solves your problem.
(Another quick note: Given your main file is HTML not CSS, you will probably need to declare that content header again after the input like this:
<?php header('Content-Type: text/html; charset=utf-8'); ?>
)

How do I embed Javascript in PHP code?

I have a HTML form and PHP code. In my form, I have a textbox and a textarea. Within both, I have included the "disabled" option. I want both textboxes to remain disabled until the user decides to click the "Edit" button, in which case both textboxes should be enabled so changes can be made and the output once again saved. According to the research I have done, the only way to do this is to use javascript, so I have included the following code within my PHP;
if (isset($_POST['edit']))
{
echo "<script type=\"text/javascript\">";
echo "var oldnotes = document.getElementById('oldnotes');";
echo "oldnotes.disabled = false;";
echo "var record = document.getElementById('record');";
echo "record.disabled = false;";
echo "</script>";
}
I have also tried;
if (isset($_POST['edit']))
{
echo "<script type=\"text/javascript\">";
echo "$('#oldnotes').removeAttr('disabled')";
echo "$('#record').removeAttr('disabled')";
echo "</script>";
}
But no luck :(
I am not receiving any errors, the textboxes just remain disabled after I click the Edit button. Could anyone help me with this? Thanks in advance.

This is a better approach for these kind of problems :
if (isset($_POST['edit'])){
?>
<script type="text/javascript">
var oldnotes = document.getElementById('oldnotes');
oldnotes.disabled = '';
var record = document.getElementById('record');
record.disabled = '';
</script>
<?php
}

<?
<script language='javascript'>
(javascript code here)
</script>
?>

Try use onclick on your Button
<input type="button" name="edit" id="edit" onclick="document.getElementById('oldnotes').disabled=false; document.getElementById('record').disabled=false; return false;">
I hope it helps.

The second approach seems correct, but you're missing ; there. Also, you haven't put any newline characters.
Here's what I suggest:
<?php
if (isset($_POST['edit'])) {
?>
<script type="text/javascript">
alert("Check entry"); // Use this for checking if your script is reaching here or not
$('#oldnotes, #record').removeAttr('disabled');
</script>
<?php
}
?>
You don't need to echo the whole thing. You can simply put it out of the PHP and keep it inside a if block.
Also, I've kept the alert to check if your code structure is proper. If you're getting the alert on clicking the edit button, means you're on the right path. You just need to workout on the JS.
If not, something wrong with your edit button and the form POST

Use single quotes for script type
echo "<script type='text/javascript'>\n";
//javascript goes here
echo "</script>";

use this jquery code:
<script>
$('#oldnotes,#record').attr('disabled','')
</script>

If you want to use JS with PHP you can read here:
how to write javascript code inside php
However: it seems you want the JS to be called on it own accord upon evaluation in the php logic as per the implementation provided.
Technically speaking your JS simply states it should re-enable the controls. Rather add the JS functionality to the OnClick of the submitting control and it should work fine. This is a quicker way of testing the functionality as well.
<a href="javascript:void(0);" onclick="document.getElementById('oldnotes').disabled=false;" >
Once it is clicked it will hide the appropriate control instead of trying to write new JS on the fly that is not executed. This is tried and tested and should work fine for you.
Hope it helps!

you can always make a cookie
use document.cookie()
set the onclick=“” to changing the cookie
and make a script that runs and gets the cookie when you open the site (use window.onload = function(){/*code goes here*/})

PHP: Reading DOM info of own HTML page

So, let's say I have only one file on my server called index.php :
<!DOCTYPE html>
<html>
<body style="background-color: orange;">
<!-- On/Off button's picture -->
<?php
echo ("<img id='button' src='data/button.jpg' alt='off'/>");
?>
</body>
</html>
Now, let's say I attach a JavaScript that changes the buttons ALT to ON when I click the button at some point.
How can I read the DOM elements on this page using PHP?
So far, I've found this code:
$dom = new DOMDocument;
$dom->loadHTML($html);
$main = $dom->getElementById('alt');
but I can't figure out how I would fit that in my code above, what page should I call loadHTML() for?
How do I make PHP read DOM elements it generated using echo?

First of all, your button has id button, so searching for the id alt would return no results.
Second: The piece of php you found reads a page $html (probably file_get_contents('some_url'), puts it in the variable dom and parses it, searching for the id alt. In your case it would read your own page, breaking it down in pieces looking for that button.
Thats of no use if you can read the status with javascript on your own page. You can then pass it to php trough an ajax-call or by adding it to the url like mypage.php?button=off

$main = $dom->getElementById('button');