如果值尚未存在，则更新单元格值

I have to update the column File on the TABLE TEST. This column contains the files related to the row. Each file is separated by a |.

An example could be

ID    NAME    FILE
 1    apple   fruit.png | lemon.png

Now when I add a new file to the FILE column I use this query:

$link->query("UPDATE TEST SET File = CONCAT(File, '$dbfilename') WHERE id = '$p_id'")

where $dbfilename can be e.g. pineapple.jpg |

The problem is that, if $dbfilename is already on the File values, it will be added another time, resulting double.

How can I check if File contains already $dbfilename, and if yes, don't add id, or even don't execute the query?

This is not a good way of storing information in a database. But I'll get to that in a second. To directly answer your question, you could use this as your SQL query:

UPDATE TEST SET File = CONCAT(File, '$dbfilename')
WHERE id='$p_id'
    AND File NOT LIKE '%$dbfilename%'
    AND Lingua='$linguadilavoro'

However, this may cause some issues when one file pineapple.jpg and you try to add another-pineapple.jpg

Really, I think you should consider how this is a horribly bad approach to databases. Consider breaking the files off into a second table. For example:

# a table for the fruit names
CREATE TABLE fruits (
    id INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(250) NOT NULL,
    UNIQUE INDEX(name)
);

# a table for file names
CREATE TABLE files (
    fileid INT UNSIGNED NOT NULL DEFAULT PRIMARY KEY AUTO_INCREMENT,
    fruitid INT UNSIGNED,
    filename VARCHAR(250),
    UNIQUE INDEX(fruitid, filename)
);

# find all of the fruits with their associated files
SELECT fruits.id, fruits.name, files.filename
FROM fruits LEFT JOIN files ON fruits.id=files.fruitid

# add a file to a fruit
INSERT INTO files (fruitid, filename)
VALUES ('$fruitID', '$filename')
ON DUPLICATE KEY UPDATE fruitid=LAST_INSERT_ID(id)

I would redesign your table structure and add a new table File with the following columns instead of using a varchar field for multiple values:

Table Test
TableId, Name

Table File
FileId, TestId, FileName

You will have to select out the FILE for the id.

then use explode to break it into an array

then check use in_array to determine if it should be added or not

Here is some (untested) code for guidance

$stmt = $link->query("SELECT File File from TEST WHERE id = '$p_id'");
$rec = $stmt->fetchAssoc();
$files = explode(" | ",$rec["FILE"]);
if (!in_array($dbfilename, $files)){
    // add to FILE
} else {
    // it's already there
}