How to use a PHP variable in Inner Join MYSQL query? -

i have been trying past day mysql recognize php variable, have had no luck far.

the code:

...connect db... session_start(); //calls session stored variable $currentusr= $_session['username']; //sql query $sql= 'select users.username, books.* users        inner join userlinkbooks lb        on users.username = lb.username        inner join books        on lb.bktitle = books.title        users.username = "$currentusr"'; $result=mysqli_query($conn,$sql);                                                                 //error check if (!$result) {                                                                                       printf("error: %s\n", mysqli_error($conn));     exit();} //display row while($row=mysqli_fetch_array($result)){                                                                                                             echo "<strong>".$row['title']."</strong>".$row['description']."</br>";}    

my issue $currentusr not calling username passed. after doing error check on it, seems empty.

what not understand when use code :

$sql = "select * users `username`='$currentusr'"; 

the variable processed , works fine, calling book title's , description perfectly. also, if manually type in: users.username = "bill"'; works fine.

some of other errors i've gotten various attempts are:

  users.username = '.'$currentusr';   error: unknown column '$currentusr' in 'where clause' 

or

  users.username = '.$currentusr;     error:  unknown column 'bill' in 'where clause' 

any appreciated. thanks

your variable in single quoted string, preventing interpolation. can try:

$sql = "select users.username, books.* users         inner join userlinkbooks lb         on users.username = lb.username         inner join books         on lb.bktitle = books.title         users.username = '" . $currentusr ."'"; 

using concatenation makes code more readable in opinion. having said that, should using parameterized queries cut down on injection issues. mysqli has such capabilities.