What does true false actually do in php - php

Question

I have seen many php function & many php scripts, I always find
function check() {
if(isset($_POST['example'])){
return true;
} else {
return false;
}
}
What does this true & false do? does false stops the query from executing further?
Actually i have a login page where I want to stop the executing if user is not found in database like:
if(mysqli_num_rows($query) !=1) {
// if result is not 1 then executing must be stop here, hello should not be echo
}
echo "hello";
further down are more script that should be executed only when result is 1
According to http://php.net/manual/en/function.return.php
-for those of you who think that using return in a script is the same as using exit note that: using return just exits the execution of the current script, exit the whole execution.
So i tried a code at my localhost
$a = 1;
if($a == 0){
echo "Its 0"; }
else{ return; }
echo "hi";
after wring return false the word hi was not executed & when i removed return false then the hi word was executed
Now i will tell you what i really want to know
$query3 =mysqli_query($connecDB, "SELECT * FROM users WHERE username='$username'");
if(mysqli_num_rows($query3) != 1)
{
$er1 = "Username doesn't exist";
}
else{
$query4 ="SELECT * FROM users WHERE email='$email'";
$result1=mysqli_query($connecDB, $query3);
if(mysqli_num_rows($result1) != 1)
{
$er1 = "Email doesn't exist";
} else {
// do this
}
}
Now you see above i have used if statement into else statement and more if statement into else which makes my php script very much complicated & very hard to understand
I just want to know what is the better way to execute script like shown below
$query3 =mysqli_query($connecDB, "SELECT * FROM users WHERE username='$username'");
if(mysqli_num_rows($query3) != 1)
{
$er1 = "Username doesn't exist";
}
else{ // stop the execution here & just print this $er1 on the page without exit or die }
i want to stop the execution because of below example
$a = 1;
if($a == 0){
echo "Its 0 "; }
else{ //if it's not 0 then it means user is not registered do not echo echo hi below }
echo "hi";
Now the hi is always executed

Answer 1

What makes the function stop executing is the Control Structure return, true or false are Boolean variables, also represented as 1 and 0 respectively.
if(function that returns a boolean !=1)
The if will only execute if function that returns a boolean is true (1)
Learn more about return and Boolean variables
Please note that mysql_* is now deprecated as of PHP7 because of security issues. It is suggested that you switch to mysqli_* or PDO extensions.