PHP PDO Named Parameters Not Working [duplicate]

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• PDO bindParam into one statement? 5 answers

Newbie to PHP here. I am attempting to update my SQLite database using:

        $insertion = $db->prepare("UPDATE roles SET ($set_string) WHERE entryDate = :entryDate AND name = :name");
        $insertion->execute($assoc_data);   

Here, $set_string is a string of attributes and parameters, which are what I want. A print_r() of $set_string yields:

name = :name,entryDate = :entryDate,development = :development,adhoc = :adhoc,mentoring = :mentoring,documentation = :documentation,codeReview = :codeReview,meetings = :meetings,learning = :learning,waste = :waste,workPrioritization = :workPrioritization,management = :management,applicationTraining = :applicationTraining

$assoc_data is an associative array with all the data that I want to update the database with:

Array ( [name] => Bobby Bob [entryDate] => 2015-08-11 [development] => 0 [adhoc] => 0 [mentoring] => 0 [documentation] => 0 [codeReview] => 0 [meetings] => 0 [learning] => 0 [waste] => 0 [workPrioritization] => 0 [management] => 0 [applicationTraining] => 100 )

My error reads:

Exception: SQLSTATE[HY000]: General error: 1 near "(": syntax error

Could someone pinpoint what the error is? I guess I don't quite understand how to use named parameters.