Form data sanitize, validation and escape php

<?php 
		$input = '<button name="submit">Delere</button>@copy;';
		echo htmlentities($input);
		echo '<br>';
	 ?>

	<?php
		//encode a html code to string for all the charecter
		$str = "A 'quote' is <b>bold</b>";
		// Outputs: A 'quote' is &lt;b&gt;bold&lt;/b&gt;
		echo htmlentities($str);
	?>

	<?php
		//encode a html code to string for some spacial charecter
		echo '<h1>htmlspecialchars function</h1><br><br>';
		$new = htmlspecialchars("<a href='test'>Test</a>", ENT_QUOTES);
		echo $new; // &lt;a href=&#039;test&#039;&gt;Test&lt;/a&gt;
	?>

	<?php
		//remove all html tags from a sting. 
		echo '<h1>strip_tags() function</h1><br><br>';
		echo strip_tags("<del>Hello</del> <b><i>world!</i></b>","<i><del>");
	?>

	<?php
	//filter all the sql injection in html form for inserting data. 
	//mysqli_real_escape_string($db_con, $sring);
	?>


	<?php
	echo '<h1>filter_var() function</h1><br><br>';
	//filter php function
	$email = 'sorwar@sorwars.me';
	if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
		echo 'The email is valid!';
	}
	else{
		echo 'The email you have entered is not valid';
	}
	?>