json_encode and json_decode in mySql misunderstanding - php

Question

I am stuck in my PHP page for a json_encode since a couple of days after having done 9999 tests.
The goal is to log on a swift app using mySql database.
So I guess i have something wrong in my query, and i dont know how to send answer of found/not found user in json_encode().
Here is my php code:
<?php
$json = file_get_contents('php://input');
$obj = json_decode($json);
// i get the email and password sent as parameter
// my swift code is working, and the next line is ok.
$logs[] = array('email' => $obj->email, 'pass' => $obj->pass);
//echo json_encode($logs); // to send back my logs (ok)
// Create connection
$con = mysqli_connect("localhost", "xx", "xx", "xx");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
//echo json_encode($logs);
// its working until here if i execute the previous line and put the rest in comment
// from the next line its making errors such :
// Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set."
$sql = "SELECT * FROM clients WHERE email = '".$logs['email']."'";
//echo json_encode($logs); //not working at this place if i put the rest in comment.
// Check if there are results
if ($result = mysqli_query($con, $sql))
{
// If so, then create a results array and a temporary one
// to hold the data
$resultArray = array();
$tempArray = array();
// Loop through each row in the result set
while($row = $result->fetch_object()) {
// Add each row into our results array
$tempArray = $row;
array_push($resultArray, $tempArray);
}
//check the corresponding password
if ( $logs['pass'] == $resultArray[4]) {
// i m not sure how to send the answer as a json
echo json_encode(true);
} else {
echo json_encode(false);
}
}
// Close connections
mysqli_close($con);
?>
If I try in php without all json stuff, its working since I put
$sql = "SELECT * FROM clients WHERE email = 'ben#test.be'";
as a test.
The following code works:
<?php
$json = file_get_contents('php://input');
$obj = json_decode($json);
$logs[] = array('email' => $obj->email, 'pass' => $obj->pass);
$con = mysqli_connect("localhost", "xx", "xx", "xx");
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$sql = "SELECT * FROM clients";
if ($result = mysqli_query($con, $sql))
{
$emparray = array();
while($row =mysqli_fetch_assoc($result))
{
$emparray[] = $row;
}
echo json_encode($emparray);
}
mysqli_close($con);
?>
But if I put $sql = "SELECT * FROM clients WHERE email = '$logs['email']'"; instead of $sql = "SELECT * FROM clients";
Its not working anymore. It doesn't like my conditional query. What did I misunderstood?