PHP notes for beginners

<?php
// comments: // or # or /* ... */

echo "Hello, Study Hub!";     // print text
echo 2 + 3;                   // 5
print "Hi there";             // print works too

// everything outside <?php ... ?> is sent as plain HTML
?>
<h1>This heading is normal HTML</h1>
<p>The PHP above ran on the server.</p>

<?php
$name    = "Ayush";      // string  - always in quotes
$marks   = 87;           // integer
$percent = 87.5;         // float
$passed  = true;         // boolean
$empty   = null;         // nothing yet

var_dump($marks);        // int(87)  - shows type AND value

// double quotes read variables, single quotes do not
echo "Hi $name";         // Hi Ayush
echo 'Hi $name';         // Hi $name

// joining text uses a dot, not a plus
echo "Scored: " . $marks . "/100";

// converting types
$age = (int) "15";       // 15
$str = (string) 99;      // "99"
$n   = intval("42");     // 42

// constants never change
define("SITE_NAME", "Study Hub");
const GST = 0.18;
echo SITE_NAME . " GST: " . GST;

<?php
$name  = "  Ayush Danthta  ";
$email = "study.hub09@gmail.com";

echo strlen("Study");                     // 5   - length
echo trim($name);                         // "Ayush Danthta"
echo strtoupper(trim($name));             // AYUSH DANTHTA
echo strtolower("HELLO");                 // hello
echo ucwords("learn coding today");       // Learn Coding Today
echo str_replace("gmail", "yahoo", $email);

echo strpos($email, "@");                 // 10  - position (false if missing)
echo substr($email, 0, 5);                // study
echo str_contains($email, "@");           // true (PHP 8+)

$parts = explode("@", $email);            // split into an array
echo $parts[0];                          // study.hub09
echo implode(", ", $parts);              // study.hub09, gmail.com

// formatting numbers
$total = 1177.6358;
echo number_format($total, 2);            // 1,177.64
echo round($total, 1);                    // 1177.6

// ALWAYS escape user text before printing it into HTML
echo htmlspecialchars($_GET['q'] ?? "", ENT_QUOTES, 'UTF-8');

<?php
$marks = 78;

if ($marks >= 90) {
    $grade = "A+";
} elseif ($marks >= 75) {
    $grade = "A";
} elseif ($marks >= 33) {
    $grade = "B";
} else {
    $grade = "F";
}

echo $grade;                       // A

// ternary
$label = $marks >= 33 ? "Pass" : "Fail";

// switch - compares loosely, so use === checks when it matters
switch (true) {
    case $marks >= 90: $tag = "Outstanding"; break;
    case $marks >= 33: $tag = "Passed";      break;
    default:           $tag = "Failed";
}

// match (PHP 8) is stricter and shorter
$colour = match (true) {
    $marks >= 90 => "gold",
    $marks >= 33 => "green",
    default      => "red",
};

<?php
// for - you know the count
for ($i = 1; $i <= 5; $i++) {
    echo $i . " ";
}

// while
$count = 1;
while ($count <= 3) {
    echo "Round $count";
    $count++;                       // forget this and it never ends
}

// foreach - by far the most used loop in PHP
$subjects = ["Maths", "Science", "English"];
foreach ($subjects as $subject) {
    echo "I study $subject";
}

// foreach with the key
$marksMap = ["Maths" => 78, "Science" => 91];
foreach ($marksMap as $subject => $score) {
    echo "$subject: $score";
}

// break and continue
foreach ($subjects as $s) {
    if ($s === "Science") continue;   // skip this one
    echo $s;
}

<?php $students = ["Ayush", "Riya", "Karan"]; ?>

<ul>
  <?php foreach ($students as $s): ?>
    <li><?= htmlspecialchars($s) ?></li>
  <?php endforeach; ?>
</ul>

<?php
// indexed array
$marks = [78, 91, 66];
$marks[] = 88;                     // add to the end
echo $marks[0];                    // 78
echo count($marks);                // 4

// associative array - named keys (this is what comes back from a database)
$student = [
    "name"  => "Ayush",
    "marks" => ["Maths" => 78, "Science" => 91],
    "city"  => "Shimla",
];

echo $student["name"];                     // Ayush
echo $student["marks"]["Science"];         // 91
$student["phone"] = "99999-00000";         // add a key
echo $student["phone"] ?? "none";          // safe read

// useful functions
sort($marks);                      // sorts in place, lowest first
rsort($marks);                     // highest first
$unique = array_unique([1, 1, 2, 3]);   // remove duplicates
$total  = array_sum([78, 91, 66]); // 235
$avg    = array_sum($marks) / count($marks);

print_r($marks);                   // readable dump while debugging
var_dump($student);                // full dump with types

// array of arrays - the shape of every database result
$classMarks = [
    ["name" => "Ayush", "marks" => [78, 91, 66]],
    ["name" => "Riya",  "marks" => [92, 88, 95]],
];

foreach ($classMarks as $s) {
    $avg = array_sum($s["marks"]) / count($s["marks"]);
    printf("%-6s %6.1f\n", $s["name"], $avg);
}

// --- the functions you will use all the time ---
$nums = [1, 2, 3, 4, 5];
$doubled = array_map(fn($n) => $n * 2, $nums);
$evens   = array_filter($nums, fn($n) => $n % 2 === 0);
$sum     = array_reduce($nums, fn($a, $b) => $a + $b, 0);
print_r($doubled);                 // [2,4,6,8,10]
echo $sum;                         // 15

// is it in the array?
var_dump(in_array("Science", ["Maths", "Science"]));   // true
print_r(array_keys($student));

<?php
function greet(string $name): string {
    return "Hello, $name!";
}

echo greet("Ayush");

// default and typed parameters
function power(int $base, int $exponent = 2): int {
    return $base ** $exponent;
}

echo power(5);        // 25
echo power(2, 10);    // 1024

// arrow function - short one-liners
$double = fn($n) => $n * 2;
echo $double(7);      // 14

// closure that remembers a value from outside
$rate = 0.18;
$withGst = function ($amount) use ($rate) {
    return $amount + $amount * $rate;
};
echo $withGst(1000);  // 1180

// functions can return an array when you need several values
function stats(array $nums): array {
    return [
        "count" => count($nums),
        "total" => array_sum($nums),
        "avg"   => array_sum($nums) / max(count($nums), 1),
    ];
}

$s = stats([78, 91, 66]);
echo "Average: " . round($s["avg"], 1);

<?php
require_once "header.php";      // crashes if missing (use this)
// include "header.php";        // only warns if missing

echo "Page content here";

require __DIR__ . "/footer.php";  // __DIR__ keeps paths reliable

<form action="save.php" method="post">
  <input name="student" placeholder="Student name" required>
  <input name="marks" type="number" min="0" max="100">
  <textarea name="note"></textarea>
  <button type="submit">Save</button>
</form>

<?php
// Only run when the form was actually submitted
if ($_SERVER["REQUEST_METHOD"] !== "POST") {
    http_response_code(405);
    exit("Method not allowed");
}

// Read, trim, then validate. Use ?? so missing keys never crash.
$student = trim($_POST["student"] ?? "");
$marks   = $_POST["marks"] ?? "";
$note    = trim($_POST["note"] ?? "");

$errors = [];

if ($student === "") {
    $errors[] = "Student name is required.";
}

if (!is_numeric($marks) || $marks < 0 || $marks > 100) {
    $errors[] = "Marks must be a number between 0 and 100.";
}

if ($errors) {
    foreach ($errors as $e) {
        echo "<p class=\"error\">" . htmlspecialchars($e) . "</p>";
    }
    exit;
}

// Safe to use now - and always escape when printing back to HTML
echo "Saved " . htmlspecialchars($student) . " with " . (int) $marks . " marks.";
?>

<?php
session_start();

if ($_SERVER["REQUEST_METHOD"] === "POST") {
    $user = trim($_POST["user"] ?? "");
    $pass = $_POST["pass"] ?? "";

    // In a real app: fetch the row from the database, then use
    // password_verify($pass, $row["password_hash"]).
    // Never compare plain-text passwords.

    if ($user !== "" && $pass !== "") {
        $_SESSION["user"] = $user;           // signed in
        header("Location: dashboard.php");   // redirect
        exit;
    }
    $error = "Enter both fields.";
}

// On another page, check the session
// if (!isset($_SESSION["user"])) { header("Location: login.php"); exit; }

Warning: Undefined array key "name" in C:\xampp\htdocs\app.php on line 12

Parse error: syntax error, unexpected '}' in app.php on line 27

Fatal error: Uncaught TypeError: greet(): Argument #1 ($name)
must be of type string, int given in app.php:5

<?php
// --- see errors while developing, hide them in production ---
ini_set("display_errors", "1");
error_reporting(E_ALL);

// --- validate instead of hoping ---
function toMarks($raw): ?int {
    if (!is_numeric($raw)) return null;
    $n = (int) $raw;
    return ($n < 0 || $n > 100) ? null : $n;
}

$marks = toMarks($_GET["m"] ?? null);
echo $marks === null ? "Bad marks value" : "Marks: $marks";

// --- catch failures instead of crashing ---
try {
    $pdo = new PDO("mysql:host=localhost;dbname=study", "root", "");
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    // prepared statement - the ONLY safe way to use user input in SQL
    $stmt = $pdo->prepare("SELECT name, marks FROM students WHERE class = ?");
    $stmt->execute([$_GET["class"] ?? ""]);
    $rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
} catch (PDOException $e) {
    error_log($e->getMessage());      // log it
    echo "Could not load data right now.";  // never show the raw error
}

<?php
$students = [
    ["name" => "Ayush", "marks" => [78, 91, 66]],
    ["name" => "Riya",  "marks" => [92, 88, 95]],
    ["name" => "Karan", "marks" => [45, 52, 60]],
];

function average(array $nums): float {
    return $nums ? array_sum($nums) / count($nums) : 0.0;
}

function gradeOf(float $avg): string {
    if ($avg >= 85) return "A";
    if ($avg >= 70) return "B";
    if ($avg >= 50) return "C";
    return "D";
}

$all = [];
foreach ($students as $s) {
    $avg = average($s["marks"]);
    $all[] = $avg;
    printf(
        "%-6s %6.1f   grade %s\n",
        htmlspecialchars($s["name"]),
        $avg,
        gradeOf($avg)
    );
}

echo str_repeat("=", 34) . "\n";
printf("Class average: %.1f\n", average($all));