Description:
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PDO constructor would fail to understand you give database name, provided you pass $dsn like '...;dbname =xyz;...', i.e. in case there's a whitespace in between "dbname" and "=".

Test script:
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$pdo = new \PDO("mysql:host=localhost;port=3306;dbname =$dbname;charset=utf8", $username, $password);

$sth = $pdo->query("select * from $tablename");

var_dump($a->errorInfo());

Actual result:
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// var_dump dsn:
string 'mysql:host=localhost;port=3306;dbname =mydatabase;charset=utf8' (length=66)

// var_dump username
string 'theuser' (length=14)

// var_dump password
string 'thepassword' (length=6)

// var_dump pdo->errorInfo()
array (size=3)
  0 => string '3D000' (length=5)
  1 => int 1046
  2 => string 'No database selected' (length=20)