Construyendo sentencias UPDATE usando arreglos asociativos en PHP

public function update($tableName, $values, $conditions = array()) {
    if (empty($values)) {
        throw new Exception('Nothing to update');
    }
    $valueStrings = array();
    foreach ($values as $name => $value) {
        $valueStrings[] = $name . ' = :' . $name;
    }
    $conditionStrings = array();
    foreach ($conditions as $column => $value) {
        $conditionString = $column;
        $conditionString .= is_array($value)
            ? ('IN ("' . implode('","', $value) . '")')
            : (' = "' . $value . '"')
        ;
        $conditionStrings[] = $conditionString;
    }
    $sql = 'UPDATE ' . $tableName
        . ' SET ' . implode(', ', $valueStrings)
        . ' WHERE ' . implode(' AND ', $conditionStrings)
    ;
    // execute query
}

Pero en realidad deberías usar un ORM para eso:

Doctrine 2:Actualizar consulta con el generador de consultas