<?php
$student_name = $_POST['student_name'];
$student_surname = $_POST['student_surname'];
 
$query = "SELECT * FROM student WHERE student_name='".$student_name."' AND student_surname='".$student_surname."' ";
?>

<?php
$student_name = $_POST['student_name'];
$student_surname = $_POST['student_surname'];
 
$query = "SELECT * FROM student WHERE student_name='{$student_name}' AND student_surname='{$student_surname}' ";
?>

<?php
$student_name = $_POST['student_name'];
$student_surname = $_POST['student_surname'];
 
$query = "SELECT * FROM student WHERE student_name='$student_name' AND student_surname='$student_surname' ";
?>

<?php
$student_name = $_POST['student_name'];
$student_surname = $_POST['student_surname'];
 
$query = "SELECT * FROM student WHERE student_name='";
$query .= $student_name ;
$query .= "' AND student_surname='";
$query .= $student_surname;
$query .= "'"; 
?>

<?php
$student_name = $_POST['student_name'];
$student_surname = $_POST['student_surname'];
 
$query = sprintf("SELECT * FROM student WHERE student_name='%s' AND student_surname='%s' ", $student_name, $student_surname);
?>