$namespaces = array_filter( $namespaces, function ( $k, $v ) {
                        wfDebug( "via array_filter: " . gettype( $v ) . " : " . gettype( NS_MAIN ) );
                        return true; // $v !== NS_MAIN;
                }, ARRAY_FILTER_USE_BOTH );
                wfDebug( "VIA reset: " . gettype( reset( $namespaces ) ) );
                wfDebug( "DUMP:" . print_r( $namespaces, true ) );
                foreach ( $namespaces as $k => $v ) {
                        wfDebug( "$k => $v | " . NS_MAIN );
                        wfDebug( "via foreach:" . gettype( $v ) . " : " . gettype( NS_MAIN ) );
                        if ( $v === NS_MAIN ) {
                                wfDebug( "UNSET" . print_r($namespaces, true) );
                                unset( $namespaces[$k] );
                        }
                }
                wfDebug( print_r($namespaces, true) );

will output:
2017-12-06 13:59:20 cirrustestwiki wfDebug DEBUG: via array_filter: integer : integer  
2017-12-06 13:59:20 cirrustestwiki wfDebug DEBUG: VIA reset: string  
2017-12-06 13:59:20 cirrustestwiki wfDebug DEBUG: DUMP:Array
(
    [0] => 0
)  
2017-12-06 13:59:20 cirrustestwiki wfDebug DEBUG: 0 => 0 | 0  
2017-12-06 13:59:20 cirrustestwiki wfDebug DEBUG: via foreach:string : integer  
2017-12-06 13:59:20 cirrustestwiki wfDebug DEBUG: Array
(
    [0] => 0
)