PHP | Warning: A non-numeric value encountered

This error occurs when you are trying to do a mathematical operation on an integer with a nun-numeric value or a string. 

Info: New E_WARNING and E_NOTICE errors have been introduced when invalid strings are coerced using operators expecting numbers (+ - * / ** % << >> | & ^) or their assignment equivalents. An E_NOTICE is emitted when the string begins with a numeric value but contains trailing non-numeric characters, and an E_WARNING is emitted when the string does not contain a numeric value.

You can solve the problem by just casting the thing into the number.

<?php
// casting the value
$total = (int)$value_1 + (int)$value_2;
echo $total;
?>

OR you can do some logic if you are taking value from the user, it is good practice to check it before doing any operation.

<?php
// Form value
$value_1 = $_POST['value1'];
$value_2 = $_POST['value2'];

// is_numeric function is use to change the value is numeric or not
if (is_numeric($value_1) && is_numeric($value_2)) {
$total = $value_1 + $value_2;
echo $total;
} else {
// do some error handling...
}
?>