Handling inputs' errors - that is one way to go

example.php

<?php 
$con = mysqli_connect( 'localhost' , 'root' , '' , 'market' );

// Inputs from post
$inputs = filter_input_array( INPUT_POST, $_POST, [
		'kind' => FILTER_SANITIZE_STRING,
		'salary' => FILTER_SANITIZE_NUMBER_FLOAT,
		'advantage' => FILTER_SANITIZE_STRING,
] );

foreach ( $inputs as $input_name => $input_value )
{
	// redirect if there are any invalid value
	// Null values will be seen as empty value
	if ( empty( $input_value ) )
		safe_redirect( 'error_'. $input_name );
}

// create variables from inputs array in runtime
extract( $inputs );

// File input validation
$file = isset( $_FILES['file'] ) ? $_FILES['file'] : null;
if ( !$file )
	safe_redirect( 'error_file_messing' );

// parse file data type
// return false if the file is not an image
$file_info = getimagesize( $file['tmp_name'] );
if ( !$file_info )
	safe_redirect( 'error_image_file' );

// check image type
if ( !in_array( $file_info['mime'], [ 'image/jpeg', 'image/jpg', 'image/pjpeg', 'image/png' ] ) )
	safe_redirect( 'error_image_type' );

// no file size !!!!! it is your choice

// generate unique file name to save with to void conflicts and file overwrite
$file_name = uniqid( 'img_' ) .'.'. pathinfo( $file['name'], PATHINFO_EXTENSION );

// save the file
$save_file = move_uploaded_file( $file['tmp_name'], 'image/'. $file_name );
if ( !$save_file )
	safe_redirect( 'error_saving_file' );

// DB statement
$stmt = mysqli_prepare( $con, "INSERT INTO mobile VALUES ( '', ?, ?, ?, ? )" );
mysqli_stmt_bind_param( $stmt, "sdss", $kind, $salary, $advantage, $file_name );

// run command
if ( !mysqli_stmt_execute( $stmt ) )
	safe_redirect( 'error_db' );

// all success
safe_redirect( 'data_inserted' );

/**
 * Redirect to main page
 * @param  string $msg
 * @return void
 */
function safe_redirect( $msg )
{
	header( 'location: main1.php?msg='. $msg, true, 302 );
	die();
}