mysql_fetch_array()/mysql_fetch_assoc()/mysql_fetch_row()/mysql_num_rows etc... expects parameter 1 to be resource

A query may fail for various reasons in which case both the mysql_* and the mysqli extension will return false from their respective query functions/methods. You need to test for that error condition and handle it accordingly.

mysql_* extension:

NOTE The mysql_ functions are deprecated and have been removed in php version 7.

Check $result before passing it to mysql_fetch_array. You'll find that it's false because the query failed. See the mysql_query documentation for possible return values and suggestions for how to deal with them.

$username = mysql_real_escape_string($_POST['username']);$password = $_POST['password'];$result = mysql_query("SELECT * FROM Users WHERE UserName LIKE '$username'");if($result === FALSE) {     die(mysql_error()); // TODO: better error handling}while($row = mysql_fetch_array($result)){    echo $row['FirstName'];}

mysqli extension
procedural style:

$username = mysqli_real_escape_string($mysqli, $_POST['username']);$result = mysqli_query($mysqli, "SELECT * FROM Users WHERE UserName LIKE '$username'");// mysqli_query returns false if something went wrong with the queryif($result === FALSE) {     yourErrorHandler(mysqli_error($mysqli));}else {    // as of php 5.4 mysqli_result implements Traversable, so you can use it with foreach    foreach( $result as $row ) {        ...

oo-style:

$username = $mysqli->escape_string($_POST['username']);$result = $mysqli->query("SELECT * FROM Users WHERE UserName LIKE '$username'");if($result === FALSE) {     yourErrorHandler($mysqli->error); // or $mysqli->error_list}else {    // as of php 5.4 mysqli_result implements Traversable, so you can use it with foreach    foreach( $result as $row ) {      ...

using a prepared statement:

$stmt = $mysqli->prepare('SELECT * FROM Users WHERE UserName LIKE ?');if ( !$stmt ) {    yourErrorHandler($mysqli->error); // or $mysqli->error_list}else if ( !$stmt->bind_param('s', $_POST['username']) ) {    yourErrorHandler($stmt->error); // or $stmt->error_list}else if ( !$stmt->execute() ) {    yourErrorHandler($stmt->error); // or $stmt->error_list}else {    $result = $stmt->get_result();    // as of php 5.4 mysqli_result implements Traversable, so you can use it with foreach    foreach( $result as $row ) {      ...

These examples only illustrate what should be done (error handling), not how to do it. Production code shouldn't use or die when outputting HTML, else it will (at the very least) generate invalid HTML. Also, database error messages shouldn't be displayed to non-admin users, as it discloses too much information.

This error message is displayed when you have an error in your query which caused it to fail. It will manifest itself when using:

• mysql_fetch_array/mysqli_fetch_array()
• mysql_fetch_assoc()/mysqli_fetch_assoc()
• mysql_num_rows()/mysqli_num_rows()

Note: This error does not appear if no rows are affected by your query. Only a query with an invalid syntax will generate this error.

Troubleshooting Steps

• Make sure you have your development server configured to display all errors. You can do this by placing this at the top of your files or in your config file: error_reporting(-1);. If you have any syntax errors this will point them out to you.

• Use mysql_error(). mysql_error() will report any errors MySQL encountered while performing your query.

  Sample usage:

  mysql_connect($host, $username, $password) or die("cannot connect"); mysql_select_db($db_name) or die("cannot select DB");$sql = "SELECT * FROM table_name";$result = mysql_query($sql);if (false === $result) {    echo mysql_error();}

• Run your query from the MySQL command line or a tool like phpMyAdmin. If you have a syntax error in your query this will tell you what it is.

• Make sure your quotes are correct. A missing quote around the query or a value can cause a query to fail.

• Make sure you are escaping your values. Quotes in your query can cause a query to fail (and also leave you open to SQL injections). Use mysql_real_escape_string() to escape your input.

• Make sure you are not mixing mysqli_* and mysql_* functions. They are not the same thing and cannot be used together. (If you're going to choose one or the other stick with mysqli_*. See below for why.)

Other tips

mysql_* functions should not be used for new code. They are no longer maintained and the community has begun the deprecation process. Instead you should learn about prepared statements and use either PDO or MySQLi. If you can't decide, this article will help to choose. If you care to learn, here is good PDO tutorial.

Error occurred here was due to the use of single quotes ('). You can put your query like this:

mysql_query("SELECT * FROM Users WHERE UserName LIKE '".mysql_real_escape_string ($username)."'");

It's using mysql_real_escape_string for prevention of SQL injection. Though we should use MySQLi or PDO_MYSQL extension for upgraded version of PHP (PHP 5.5.0 and later), but for older versions mysql_real_escape_string will do the trick.