by php and html please, when click on submit, if there is error in any input value, print that error, else print all data that the user entered in the FORM

By PHP, how can I print the error if it's occurred when the user entered wrong value in FORM, like price should be numbers, not letters, also name, city, address ets. should be not empty. So, if there is error in any field in the form, print that error like PLEASE INPUT PRICE, or, PLEASE CHOSE ONE ITEM, otherwise if there is no error print the data that the use entered