You're missing a ; on line 1.

$display_box_content = '<div style="box-sizing: border-box; color: rgb(37, 37, 37); font-family: Axiforma-Regular; font-size: 16px;"></div>'; //added a ; here
$display_box_content = str_replace('"', "'", $display_box_content);   

This code actually works; Please see 3v4l

First of all, it is always better to check for the unwanted characters and get back to the user, than silently stripping them. Say, a user added a quote to their password, you removed it, and so they won't be able to login at all! So it's better to check and tell the user instead:

if (!ctype_alnum($username)) { 
    $errors[] = "Only letters and numbers allowed in username";
}
...
if ($errors) {
    echo "You've got some errors, please fix them: ". implode("<br>", $errors);
} else {
    // proceed with normal flow

But in this specific case I would advise against replacing any characters. Imagine Shaquille "Shaq" O'Neal is going to register on your site. What's the point in stripping him of all those quotes? Let alone the password, where use of punctuation is strongly encouraged.

After all, those quotes don't do any harm, if you properly handle them.

• for HTML, simply use htmlspecialchars() with ENT_QUOTES attribute:

    Your username is: <?= htmlspecialchars($username, ENT_QUOTES) ?><br />
  

• for SQL, use prepared statements to avoid any problems with quotes

But just for sake of literal answer, here is how to remove quotes (or any other characters and even multi-character substrings) in PHP

$yourVariable = "scor\"pi'on";
$substringsToRemove = ['\'', '"'];
$yourVariable = str_replace($substringsToRemove, "", $yourVariable);

str_replace('"', "'", $text);

or Re-assign it

$text = str_replace('"', "'", $text);

Try Function STR_REPLACE

Syntax;

str_replace(find,replace,string,count)

EX:

$string = str_replace("'","''","'test");

Update: I'd agree with others that the following is an easier-to-read alternative for most folks:

$page = str_replace("'", '"', $page);

My original answer:

$page = str_replace(chr(39), chr(34), $page);

I didn't have a problem with your code, my test is below:

<?php

    $input = '"This" is a '."'".'String'."'";
    echo $input.'<br />';
    //Echos  "This" is a 'String'

    $output = str_replace('"','\"',$input);
    $output = str_replace("'","\'",$output);
    echo $output;
    //Echos  \"This\" is a \'String\'

Edited

Irrelevant now, OP figured it out :D

Or:

$con = str_replace(chr(34), chr(39), $content);

PHP's DOMDocument class does this automatically for you. All you need to do is load the HTML and save it back. This might work better than a regex as it's not dependent on the markup format. The downside is that it will fix all the HTML, not just the quotes around attributes that begin with a particular domain.

$dom = new DOMDocument;
libxml_use_internal_errors(TRUE);
$dom->loadHTML($html);
$result = $dom->saveHTML();

Demo

I don't see a use-case for replacing just some particular piece of HTML markup. But I don't know what you're trying to accomplish, so I will offer a regex solution as well:

preg_replace("#\b\w+\s*='((?=http://www\.website\.com)[^']+)'#is",'"$1"',$html);

This uses a positive lookahead to check of the attribute value begins with with the URL you've defined.

Regex101 demo

You haven't provided much context for your question so it's difficult to know exactly how to help, but lets say you have a variable called $title with double quotes in it, you can replace them like this:

$title = str_replace('"', "'", $title);

Use str_replace:

$string = '<a href="""">test</a>';
$string = str_replace('""', '"', $string);

echo $string;

// Outputs: <a href="">test</a>