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

or Re-assign it

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

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);

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

$value = str_replace(""", "", $value);

Thanks AbraCadaver

The MS smart quotes can be removed with this function.

function convert_smart_quotes($string) 
{ 
    $search = array(chr(145), 
                    chr(146), 
                    chr(147), 
                    chr(148), 
                    chr(151)); 

    $replace = array("'", 
                     "'", 
                     '"', 
                     '"', 
                     '-'); 

    return str_replace($search, $replace, $string); 
} 

Or add them to your code:

str_replace(array('<','>',')','\$','(', '?', '.', ',' ,'!', '-', '+', '/', '\*', '\\', '"', chr(145), chr(146), chr(147), chr(148)), " ", $text)

http://shiflett.org/blog/2005/oct/convert-smart-quotes-with-php

str_replace()

echo str_replace('"', '', $a);

You really shouldn't do it. You should use PDO and prepared statements or at least mysqli and mysqli_real_escape_string. Using addslashes to insert data to database it's very bad idea.

EDIT

And you shouldn't use mysql functions (I see you tried in your question comment) because they are deprecated already. Use mysqli functions if you don't want to use PDO

You need to escape double quotes in your string literal. There's also an umatched left-parenthesis you need to remove:

<?php
$comments = str_replace('"', "'", "6:00 pm , practiced \"Zen' flying  and sit carving (one leg down) and back carving and sit-to-sit front flip (weight require slower wind speed)");

echo $comments;
?>

Output:

6:00 pm , practiced 'Zen' flying  and sit carving (one leg down) and back carving and sit-to-sit front flip (weight require slower wind speed)

Edit: Since you've posted more code I see what is going on. Try the following code. It will work unless the content has "LOG_COMMENTS\n" in it somewhere.

$time[] = array(
    "entry_id" => "{entry_id}", 
    "tunnel" => $tunnel,
    "tunnel_id" => $tunnel_id,
    "entry_date" => "{entry_date}", 
    "log_time" => "{log_time}", 
    "log_video" => "{log_video}", 
    "log_comments" => <<<LOG_COMMENTS
{log_comments}
LOG_COMMENTS
);

Still, this is a very poor design. Is there a reason the CMS can't save data into a database or a plain text file?

Using your current str_replace method:

$FileName = str_replace("'", "", $UserInput);

While it's hard to see, the first argument is a double quote followed by a single quote followed by a double quote. The second argument is two double quotes with nothing in between.

With str_replace, you could even have an array of strings you want to remove entirely:

$remove[] = "'";
$remove[] = '"';
$remove[] = "-"; // just as another example

$FileName = str_replace( $remove, "", $UserInput );

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