Cannot access empty property error in php - php

Question

I have some text stored in a table. I want to replace all the expressions that look like this : [%parameter_name] with the corresponding value stored in another table.
All the parameters and in which column I must fetch their value are stored in a third column.
I tried to make a function to put all the needed patterns and their value in an array. However, it gives me a fatal error. Why?
Here is the code :
$search = $db->prepare("
SELECT *
FROM table
WHERE id = ?");
$search->execute(array($id));
$search->setFetchMode(PDO::FETCH_OBJ);
while($result = $search->fetch()){
$search2 = $db->prepare("
SELECT parameter_name,
read_from
FROM pattern
WHERE type = ?");
$search2->execute(array(strtolower($result->type)));
$search2->setFetchMode(PDO::FETCH_OBJ);
while($pattern = $search2->fetch()){
array_push($patterns, '/\[%'.$result->type.'\.'.$pattern->parameter_name.'\]/i');
$column = explode(',', $pattern->read_from);
if(count($column) == 1){
$col = $column[0];
// ERROR ON THE LINE BELOW
$value = $result->$col;
}
else{
$value = 0;
foreach($column as $col){
// BUT NOT ON THE ONE BELOW
$value += $result->$col;
}
}
array_push($replacements, $value);
}
}