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);

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

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);

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

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

Hope this helps.

<?php

    //left double quote 
         $temp = str_replace("&ldquo;","\"",$temp);
         $temp = str_replace("&#8220;","\"",$temp);

        //right double quote 
         $temp = str_replace("&#8221;","\"",$temp);
         $temp = str_replace("&rdquo;","\"",$temp);
    ?>

Use str_replace:

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

echo $string;

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

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?

Better yet, try to use ' instead of " for the outer quote so you don't have to escape it at all. And concatenate $raiz with your string. The problem is most likely characters that should be escaped in html.

Try this, this works (see this code exactly here):

$changeli = 'li id="head'.$raiz.'" class="head"';
$changeli2 = 'li id="foot'.$raiz.'" class="foot"';

// value of $raiz is "-all" at this point (for clarity of code)
echo $changeli; // result should be li id="head-all" class="head"
echo $changeli2; // result should be li id="foot-all" class="foot"

$footer = htmlspecialchars_decode(str_replace(htmlspecialchars($changeli), htmlspecialchars($changeli2), htmlspecialchars($footer)));

There are many characters that look like quotation marks, most of them are used infrequently. The ones that are used more often are these three:

"   U+0022 QUOTATION MARK
“   U+201C LEFT DOUBLE QUOTATION MARK
”   U+201D RIGHT DOUBLE QUOTATION MARK

Some rarer ones are FULLWIDTH QUOTATION MARK, the DITTO MARK, the DOUBLE PRIME, the DOUBLE PRIME QUOTATION MARK, and so on. The Unicode.org "confusables" tool finds 15 characters similar to ".

Why don't you copy and paste the offending character here so we can identify it? Or you could use the HEX function to get the hexadecimal encoding of the character, that's another way of identifying it.

Update The unicode.org confusables utility seems to be down, but the data is available as a text file. The current list of characters that are "confusable" with double quote are:

1CD3 ;  0027 0027 ; MA  #* ( ᳓ → '' ) VEDIC SIGN NIHSHVASA → APOSTROPHE, APOSTROPHE # →″→→"→
0022 ;  0027 0027 ; MA  #* ( " → '' ) QUOTATION MARK → APOSTROPHE, APOSTROPHE   # 
FF02 ;  0027 0027 ; MA  #* ( ＂ → '' ) FULLWIDTH QUOTATION MARK → APOSTROPHE, APOSTROPHE # →”→→"→
201C ;  0027 0027 ; MA  #* ( “ → '' ) LEFT DOUBLE QUOTATION MARK → APOSTROPHE, APOSTROPHE   # →"→
201D ;  0027 0027 ; MA  #* ( ” → '' ) RIGHT DOUBLE QUOTATION MARK → APOSTROPHE, APOSTROPHE  # →"→
201F ;  0027 0027 ; MA  #* ( ‟ → '' ) DOUBLE HIGH-REVERSED-9 QUOTATION MARK → APOSTROPHE, APOSTROPHE    # →“→→"→
2033 ;  0027 0027 ; MA  #* ( ″ → '' ) DOUBLE PRIME → APOSTROPHE, APOSTROPHE # →"→
2036 ;  0027 0027 ; MA  #* ( ‶ → '' ) REVERSED DOUBLE PRIME → APOSTROPHE, APOSTROPHE    # →‵‵→
3003 ;  0027 0027 ; MA  #* ( 〃 → '' ) DITTO MARK → APOSTROPHE, APOSTROPHE   # →″→→"→
05F4 ;  0027 0027 ; MA  #* ( ‎״‎ → '' ) HEBREW PUNCTUATION GERSHAYIM → APOSTROPHE, APOSTROPHE   # →"→
02DD ;  0027 0027 ; MA  #* ( ˝ → '' ) DOUBLE ACUTE ACCENT → APOSTROPHE, APOSTROPHE  # →"→
02BA ;  0027 0027 ; MA  # ( ʺ → '' ) MODIFIER LETTER DOUBLE PRIME → APOSTROPHE, APOSTROPHE  # →"→
02F6 ;  0027 0027 ; MA  #* ( ˶ → '' ) MODIFIER LETTER MIDDLE DOUBLE ACUTE ACCENT → APOSTROPHE, APOSTROPHE   # →˝→→"→
02EE ;  0027 0027 ; MA  # ( ˮ → '' ) MODIFIER LETTER DOUBLE APOSTROPHE → APOSTROPHE, APOSTROPHE # →″→→"→
05F2 ;  0027 0027 ; MA  # ( ‎ײ‎ → '' ) HEBREW LIGATURE YIDDISH DOUBLE YOD → APOSTROPHE, APOSTROPHE  # →‎יי‎→

str_replace()

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