Simple PHP query not working when i use a variabel

Okay this has been driving me crazy for the last few hours! So simple but it just doesn’t work and I really don’t see what’s going wrong here.

A simple sql query doesn’t work when using a variable, while with identical plain text it does.

To give a clear example, I have the code below with the output of all possible things that can go wrong.

<?php
        $sRecipient     = $data[0];
        $sTitle         = $titel;
        $sBody          = $bericht;
        $sSender        = $_SESSION['username'];
        $aAttachments   = null;
        $iType          = 0;

        echo "Output $sRecipient: ".$sRecipient."";

        $pdo = new PDO(DB_CONFIG_DSN, DB_CONFIG_USER, DB_CONFIG_PW);   
                        
        $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);     
        $pdo->exec('SET NAMES "utf8"');     

        // NOT WORKING ???????????
        $sql_user_details_var = "SELECT gebruikersnaam, internnummer FROM `leerlingen` WHERE `gebruikersnaam`='".$sRecipient."'";
        $user_details_result = $pdo->query($sql_user_details_var);     
        $user_details_result = $pdo->query($sql_user_details_var)->fetch(PDO::FETCH_ASSOC);

        // WORKING!
        $sql_user_details_text = "SELECT gebruikersnaam, internnummer FROM `leerlingen` WHERE `gebruikersnaam`='[email protected]'";
        $user_details_result = $pdo->query($sql_user_details_text);     
        $user_details_result = $pdo->query($sql_user_details_text)->fetch(PDO::FETCH_ASSOC);                       

        echo "<br /><br />SQL with text: ".$sql_user_details_var."<br />";
        echo "<br />SQL with var: ".$sql_user_details_text."<br /><br />";

        // FOUND THE ROW!
        $result_text    = $pdo->query('select count(*) from `leerlingen` WHERE `gebruikersnaam`="[email protected]"')->fetchColumn();

        // NOTHING FOUND ???????????
        $result_var     = $pdo->query('select count(*) from `leerlingen` WHERE `gebruikersnaam`="'.$sRecipient.'"')->fetchColumn();

        echo "Result text: ".$result_text."<br />";
        echo "Result var: ".$result_var."<br />";
?>

This is the output:

Output $sRecipient: [email protected]

SQL with text: SELECT gebruikersnaam, internnummer FROM `leerlingen` WHERE `gebruikersnaam`='[email protected]'

SQL with var: SELECT gebruikersnaam, internnummer FROM `leerlingen` WHERE `gebruikersnaam`='[email protected]'

Result text: 1
Result var: 0

Does anyone have any idea what I’m overlooking?
Tried everything, although you can’t go wrong with this, can you?