How and when to use isset() and empty() correctly?

According to the isset() documentation :

Returns trueif the variable exists and has a value other than null, falseotherwise.

For example:

$var = ''; // Cadena vacía

if (isset($var)) { // <= true
    ... 
}

It will always return to you trueeven if the variable is empty and it should be checked with the function empty():

$var = ''; // Cadena vacía

if (!empty($var)) { // <= false

    // No está vacía (true)

} else {

    // Está vacía (false)
}

And when should you use isset()?

As long as you want to check that the variable exists, a good example would be if you want to know if a form has been submitted :

<form method="post" action="">
    ...
    <input type="text" name="campo1" value="">
    <input type="submit" name="submit" value="Enviar formulario">
</form>

if (isset($_POST['submit']) {
  ...
}

According to the empty() documentation :

Returns falsewhether the variable exists and has a non-empty, non-zero value. Otherwise it returns true.

With empty()the following expressions are considered as empty and return true:

"" (una cadena vacía)
0 (0 como un integer)
0.0 (0 como un float)
"0" (0 como un string)
NULL
FALSE
array() (un array vacío)
$var; (una variable declarada, pero sin un valor)

Important to know, that a warning is not generated if the variable does not exist.

This means that it empty()is essentially the concise equivalent of:

!isset($var) || $var == false.

So if it would be redundant if we check the same variable with isset()and empty()at the same time.

An example how it can be used isset()and !empty()at the same time when you want to receive data from a form:

<form method="post" action="">    
    <input type="text" name="campo1" value="">
    ...
    ...
    <input type="submit" name="submit" value="Enviar formulario">
</form>

if (isset($_POST['submit']) {

    if (!empty($_POST['campo1'])) {
        ...
    }
}

Isset

Isset you use it when you want to know

if a variable is defined and is not NULL..

Example, file1.php

 if ( isset($_POST['submit']))
    {
      // Acciones
    }

This can be useful if you want to validate that a file receives POST variables, since you could access by url localhost/myfolder/file1.php , if you access like this you are not sending any value by POST, so it does not enter the If.

Another example:

$var = '';

// Esto evaluará a TRUE así que el texto se imprimirá.
if (isset($var)) {
    echo "Esta variable está definida, así que se imprimirá";
}

Source: isset documentation

Empty

Empty is to know if a variable is empty.

"A variable is considered empty if it does not exist or if its value is equal to FALSE. empty() does not generate a warning if the variable does not exist."

$var = 0;

// Se evalúa a true ya que $var está vacia
if (empty($var)) {
    echo '$var es o bien 0, vacía, o no se encuentra definida en absoluto';
}

What PHP considers as empty:

• "" (an empty string)
• 0 (0 as an integer)
• 0.0 (0 as a float)
• "0" (0 as a string)
• NULL
• FALSE
• array() (an empty array)
• $var; (a variable declared, but without a value)

Source: empty documentation

Is it redundant to use isset together with empty?

No, since there can be a variable defined but whose value is 0, then it is necessary to first evaluate that it is not null, and if it is not null evaluate that it is not empty in case you want to return a message to the user indicating that he must enter a value or if you entered it, indicate that it cannot be 0 or a space, to name two examples (the format in general).

$var = 0;

if (isset($var))
  echo 'La variable está definida';

if (empty($var))
  echo 'La variable está definida pero está vacía';

Conclusion: isset and empty evaluate a variable differently . Isset if a variable is null, empty if it is empty and can be NULL or any of the previous list.

Excellent in the index you can also use this form using two statements:

if (isset($_POST['$var1']) && !empty($_POST['$var1']) && (isset($_POST['$var2']) && !empty($_POST['$var2']))) {
    echo "<pre>";
    print_r($_POST);


        require_once "conexion.php";
            require_once "buscar.php";
}