мда.

if (isset($_POST['city']) && !empty($_POST['city'])) {

а это

$where = implode(' AND ', $where);

напиши так

$where = isset($where) ? implode(' AND ', $where) . ' and ' : '';

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решил сделать таким образом

if ($city = '') {
	 $zapros = "select `img`, `id`, `title`, `city`, substring(`text` from 1 for 100), `date`, `raiting`, `proverka`, `q_vote` from `date` where  (cat_id='$cat') and (proverka='1') order by date desc";
	}
	else {
	 $zapros = "select `img`, `id`, `title`, `city`, substring(`text` from 1 for 100), `date`, `raiting`, `proverka`, `q_vote` from `date` where city='$city' cat_id='$cat' and proverka = '1' order by date desc";
	}
	
	$result3 = mysql_query("".$zapros."",$db);

теперь выдает синтакчисескую ошибку

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'cat_id='1' and proverka = '1' order by date desc' at line 1

where city='$city' cat_id='$cat' and proverka

where city='$city' and cat_id='$cat' and proverka

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HTML, CSS (Bootstrap), JS(JQuery, ExtJS), PHP, MySQL, MSSql, Posgres, (TSql, BI OLAP, MDX), Mongo, Git, SVN, CodeIgnater, Symfony, Yii 2, JiRA, Redmine, Bitbucket, Composer, Rabbit MQ, Amazon (SQS, S3, Transcribe), Docker

Игорь_Vasinsky, спасибо! исправил - но все равно не работает! сделал таким образом и теперь вообще ничего не выводит

1. что в POST ?
2. распечатай запрос

exit($zapros);

и проверь его в phpMyAdmin

_____________
HTML, CSS (Bootstrap), JS(JQuery, ExtJS), PHP, MySQL, MSSql, Posgres, (TSql, BI OLAP, MDX), Mongo, Git, SVN, CodeIgnater, Symfony, Yii 2, JiRA, Redmine, Bitbucket, Composer, Rabbit MQ, Amazon (SQS, S3, Transcribe), Docker

Игорь_Vasinsky

if (isset($_POST['city']))        {$city = $_POST['city']; if ($city == '') {unset($city);}}