PHP json_encode array(3) return false -

i have mysqli query returns multidimensional array.

i'm trying encode php array:

array(3) {   [0]=>   array(8) {     ["cod_evento"]=>     string(1) "3"     ["titulo"]=>     string(20) "fiesta de cumpleaños"     ["descripcion"]=>     string(23) "mi fiesta de cumpleaños"     ["puntuacion"]=>     string(1) "1"     ["direccion"]=>     string(13) "c/ falsa 1234"     ["imagen"]=>     string(115) "http://static.guim.co.uk/sys-images/guardian/pix/pictures/2014/4/11/1397210130748/spring-lamb.-image-shot-2-011.jpg"     ["fecha"]=>     string(19) "2015-01-01 00:00:00"     ["cod_usuario"]=>     string(1) "1"   }    [....] } 

but when try json_encode($array) returns bool(false)

when try encode array(2) or (1), works.

from docs

http://php.net/manual/en/function.json-encode.php

returns json encoded string on success or false on failure.

to see reason of failure

http://php.net/manual/en/function.json-last-error-msg.php

to answer question in comments

json_encode produce json_error_utf8 mssql-select