On a form, there is a field where you will need to perform a math operation. If the result of this operation is right, it allows the sending, otherwise it will not. The operation should contain two random numbers from 1 to 9.
For example: 1 + 2 = 3 (send the form).
How to do this in PHP?
You can validate the sum with JavaScript before sending (you will only submit the form if the sum is correct): by dvd
<?php
$n1 = rand(1,9);
$n2 = rand(1,9);
$soma = $n1+$n2;
?>
<form method="post" action="">
<br>
<label for="captcha">Não sou robot: <?php echo $n1." + ".$n2 ?> =</label>
<input type="text" name="captcha" id="captcha" />
<input type="hidden" name="soma" id="soma" value="<?php echo $soma ?>" />
<input type='submit' value='Enviar'>
</form>
<script>
var form = document.body.querySelector("form");
form.addEventListener("submit", function(e){
var capt = document.body.querySelector("#captcha"),
resu = document.body.querySelector("#soma");
if(resu.value == capt.value){
//envia o formulário
alert("Soma OK!");
}else{
// não envia o formulário
console.log("Soma incorreta");
e.preventDefault();
}
});
</script>