Get data from two MySQL tables in JSON using PHP if possible use joins

Well, you can create your own array to hold your results from one table.

<?php  
$db=new PDO('mysql:dbname=fvilla;host=localhost;','root','');  
$row=$db->prepare('select * from slider');  
$row->execute();//execute the query  
$json_data=array();//create the array
$result = array();
foreach($row as $rec)//foreach loop  
{  
    $json_array['id']=$rec['id'];  
    $json_array['image_url']=$rec['image_url'];  
    $json_array['link_image']=$rec['link_image'];  
    array_push($json_data,$json_array);  

}  
$result["table1"] = $json_data;

$row1=$db->prepare('select * from collections');  
$row1->execute();//execute the query  
$json_data1=array();//create the array  
foreach($row1 as $rec1)//foreach loop  
{  
    $json_array1['cid']=$rec1['id'];  
    $json_array1['cimage_url']=$rec1['image_url'];  
    $json_array1['cheading']=$rec1['heading']; 
    $json_array1['csubheading']=$rec1['subheading'];  
    array_push($json_data1,$json_array1);  
}
$result['table2'] = $json_data1;
echo json_encode($result);  
?>

Not entirely sure that this will suffice, but something like this might be what you are after:

$db=new PDO('mysql:dbname=fvilla;host=localhost;','root','');  
$output=array();

$row=$db->prepare('select * from slider');  
$row->execute();
$sliders=array();
foreach( $row as $key => $rec ){  
    $sliders[ 'slider'.$key ]=array('id' => $rec['id'],'image_url' => $rec['image_url'],'link_image' => $rec['link_image']);
} 
$output['sliders']=$sliders;
#echo json_encode( $sliders );  



$row=$db->prepare('select * from collections');  
$row->execute();
$collections=array();
foreach( $row as $key => $rec ){  
    $collections [ 'collections'.$key ]=array('cid' => $rec['id'],'cimage_url' => $rec['image_url'],'cheading' => $rec['heading'],'csubheading' => $rec['subheading'] );
}
$output['collections']=$collections;
#echo json_encode( $collections );

echo json_encode( $output );

You can use UNION ALL to get all data from both tables in a single query i.e.

$db=new PDO('mysql:dbname=fvilla;host=localhost;','root','');  
$row=$db->prepare('select id as slider_id, image_url as slider_image, heading, subheading from slider UNION ALL select id as header_id, image_url as header_image from collections');  
$row->execute();//execute the query  
$result = $row->fetchAll(PDO::FETCH_ASSOC);
$json_array = array();
foreach($result as $rec)//foreach loop  
{  
    $json_array['slider1']['id']=$rec['slider_id'];  
    $json_array['slider1']['image_url']=$rec['slider_image'];  
    $json_array['slider1']['link_image']=$rec['link_image'];   

    $json_array['slider2']['cid']=$rec['header_id'];  
    $json_array['slider2']['cimage_url']=$rec['header_image'];  
    $json_array['slider2']['cheading']=$rec['heading']; 
    $json_array['slider2']['csubheading']=$rec['subheading'];  

}
echo json_encode($json_array);

NOTE:-

Use UNION ALL to collect all data from two queries, since you have id, image_url common fields in both table so use alias as with different key as mentioned in query

Hope this works, Happy coding...