Add Additional Function to Variable

To validate email addresses (which I believe is the problem here!), you can use regular expressions. Google will find several approaches (see http://www.google.ca/search?q=php+regular+expression+email+address&sourceid=navclient-ff&ie=UTF-8&rlz=1B3GGGL_enCA223CA224).

One solution, using an array of candidate emails as shown in the question is shown in the snippet. It will

1. write each email address (candidate) from the array $existing_emails
2. append "ok" or "NO" to indicate whether it's valid.

Note that the eregi PHP function is case insensitive, so the strtolower function can be skipped.

foreach ($existing_emails as $email) {
  echo $email . " ";
  if (eregi('^[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,5}$', $email)) {
    echo "ok\n";
  }
  else {
    echo "NO\n";
  }
}

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Of course, EE has solutions as well:
For a list, see
https://www.experts-exchange.com/simpleSearch.jsp?q=php+validate+email&searchSubmit=1&cid=315&redirectURL=%2F&gSearch=0

For one candidate solution, see
https://www.experts-exchange.com/questions/21742294/php-validate-email.html?sfQueryTermInfo=1+email+php+valid

Can't I do something simpler like:

// NORMALIZE TO LOWER CASE BECAUSE you@your.org == You@YOUR.OrG IN EMAIL
$existing_emails[] = eregi('^[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,5}$', trim(strtolower($row["email_cre"])));
// WOULD THIS WORK OR..?

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Wait, let me raise the points.

Better yet, I would like to simply remove the special characters (not found in email addresses) like exclamation points, etc. from the array. So, do you have something that I can simply append to the current lower-case'ing script that will just remove specified special characters?

Points raised to 250 from 125.

Before proceeding, one cautionary question:

Is the goal to get a list of valid email addresses from a list that may include some garbage?

If you have something like emb!@you!r.org, you get a valid email address by extracting the ! characters. However,
input like this: !#@$@#!$asdfas)*&d12 will not generate a valid email address. (There's no guarantee that you will end up with a valid email address.) Is it really satisfactory to turn !#@$@#!$asdfas)*&d12 into @@asdfasd12?

Thanks for your concern. The application is only for a list of email addresses like johndoe@hiswebsite.com; but occasionally they're will be an email like johno'doe@hiswebsite.com, know what I mean? So, I just think to remove the apostrophe in such an event would be closer to an actual address.

The attached snippet extracts all characters other than
a-z
A-Z
0-9
@
.
_

Others can be added to $pattern if needed.

The checked/updated email addresses are placed in the $checked_emails array.

$pattern = '[^a-z@\.0-9_]'; // "^" means characters NOT in this list
$replacement = '';
$checked_emails = array();
 
//fill array with each checked and possibly updated $existing_emails value
//note: eregi... the "i" means case-insensitive
foreach ($existing_emails as $email) {
  $checked_emails[] = eregi_replace($pattern, $replacement, trim($email));
}

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NB trim($email) above can be replaced with $email as spaces will be extracted.

i.e.
  $checked_emails[] = eregi_replace($pattern, $replacement, trim($email));

can be
  $checked_emails[] = eregi_replace($pattern, $replacement, $email);

Is this working for you? Need clarification?

Thanks, I was just wondering about the trim function from:
$checked_emails[] = eregi_replace($pattern, $replacement, trim($email));

Doesn't it need to be like this to make it lower-case:
trim(strtolower($row["email_cre"]));

Would I do this:
$checked_emails[] = eregi_replace($pattern, $replacement, trim(strtolower($row["email_cre"])));;

Thanks for being so thorough! It helps me understand things better. Here's some points! Have a good day.

Thanks, again. Have a good day.

You're welcome!

Hey, quick question: I tried applying the code (as attached) but when I run an email like this:
!!f!@!f!!!.com

I simply submits this:
!!f!@!f!!!.com

I wanted it to be stripped of special characters (such as "!") and submit as this:
f@f.com

Can you help or should a post a new question?

		// added 
		$pattern = '[^a-z@\.0-9_]';
		$replacement = '';
		// end added
		while ($row = mysql_fetch_assoc($rs))
		{
		// NORMALIZE TO LOWER CASE BECAUSE you@your.org == You@YOUR.OrG IN EMAIL
   			// $existing_emails[] = trim(strtolower($row["email_cre"])); <--- changed from this
			$existing_emails[] = ereg_replace($pattern, $replacement, strtolower($row["email_cre"]));
		}

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