问题的出现原因：代码中使用了正则表达式替换函数preg_replace，将换行符替换成了Excel中的单元格换行符（<br style="mso-data-placement:same-cell;" />），但是由于代码中使用了mysql函数，而mysql函数在PHP 7.0版本中已被弃用，导致代码无法正常执行。

解决方法：将代码中的mysql函数替换为mysqli函数，同时修改连接数据库的方式，以适应PHP 7.0版本及以上的要求。具体修改代码如下：

<?php
/*******EDIT LINES 3-8*******/
$DB_Server = "localhost"; //MySQL Server    
$DB_Username = "root"; //MySQL Username     
$DB_Password = ""; //MySQL Password     
$DB_DBName = "survey"; //MySQL Database Name  
$DB_TBLName = "results"; //MySQL Table Name   
$filename = "survey_results"; //File Name
/*******YOU DO NOT NEED TO EDIT ANYTHING BELOW THIS LINE*******/
//create MySQL connection   
$mysqli = new mysqli($DB_Server, $DB_Username, $DB_Password, $DB_DBName);
if ($mysqli->connect_errno) {
    die("Couldn't connect to MySQL: " . $mysqli->connect_error);
}
//execute query 
$sql = "SELECT username, email, q1, q2, q3, createdat FROM $DB_TBLName";
$result = $mysqli->query($sql);
if (!$result) {
    die("Couldn't execute query: " . $mysqli->error);
}
$file_ending = "xls";
//header info for browser
header("Content-Type: application/xls");    
header("Content-Disposition: attachment; filename=$filename.xls");  
header("Pragma: no-cache"); 
header("Expires: 0");
//define separator (defines columns in excel & tabs in word)
$sep = "\t"; //tabbed character
//start of printing column names as names of MySQL fields
echo <<<EOF
    <!--- Store Excel-HTML output. --->
<cfsavecontent variable="strData">
    <table border=".5pt">
    <thead>
        <tr>
EOF;
while ($field = $result->fetch_field()) {
    echo "<th>" . $field->name . "</th>";
}
echo <<<EOF
    </thead>
    <tbody>
EOF;
//end of printing column names  
//start while loop to get data
while ($row = $result->fetch_row()) {
    echo "<tr valign=\"top\">";
    foreach ($row as $cell) {
        echo "<td>";
        if (!isset($cell)) {
            echo preg_replace("/\r\n|\n\r|\n|\r/", "<br style=\"mso-data-placement:same-cell;\" />", "NULL" . $sep);
        } elseif ($cell != "") {
            echo preg_replace("/\r\n|\n\r|\n|\r/", "<br style=\"mso-data-placement:same-cell;\" />", $cell . $sep);
        } else {
            echo preg_replace("/\r\n|\n\r|\n|\r/", "<br style=\"mso-data-placement:same-cell;\" />", "" . $sep);
        }
        echo "</td>";
    }
    echo "</tr>";
}       
echo <<<EOF
    </tbody>
    </table>
</cfsavecontent>
<!--- Set header for file attachment. --->
<cfheader
    name="content-disposition"
    value="attachment; filename=data.xls"
    />
<!--- Stream the content. --->
<cfcontent
    type="application/excel"
    variable="#ToBinary( ToBase64( strData ) )#"
    />
EOF;
?>

以上是根据给出的代码整理出的原因和解决方法。