Sobrepor valor no banco de dados

Estou criando um sistema simples de depósito porém estou tendo um problema. Sempre q faço um novo depósito, ao invés de sobrepor o antigo, cria se um novo valor no banco de dados:

<?php
$dep = $_POST['depositar']. $conn = mysql_connect('localhost', 'root', '123' ). $db = mysql_select_db('test'). $query_select = "SELECT depositar FROM dinheiro WHERE depositar = '$dep'". $select = mysql_query($query_select, $conn). $array = mysql_fetch_array($select). $saldo = $array['depositar']. $query = "INSERT INTO dinheiro (depositar) values ('$saldo' + '$dep')". $inset = mysql_query($query, $conn).?>

no banco de dados está apresentando os valores assim:

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