How to use PHP form validation functions ISSET(), empty(), is_numeric()

ISSET();——Suitable for detecting whether this parameter exists.
Definition and scope: used to test whether a variable has a value (including 0, FALSE, or an empty string, but not NULL), that is: "http://localhost/?fo=" can also pass the test, Therefore not applicable. But if the "http://localhost/" parameter does not contain the fo parameter, you can use isset to detect it. In this case, isset($_GET['fo']) returns false.
Not applicable: This function is not suitable for validating text in html forms in an efficient way. To check whether the user input text is valid, you can use empty();

empty(); - the best function to use.
Definition and scope: Used to check whether a variable has a null value: including: empty string, 0, null or false, that is: "http://localhost/?fo=" or "http://localhost/?fo =0", the results detected by empty are all true, not applicable scope: not suitable for detecting parameters that can be 0.

is_numeric(); - only suitable for detecting numbers, but if the parameter name does not exist, an error will occur, so it is not suitable for the first level of detection.
Comprehensive example:

ini_set("display_errors",1);
//ini_set("error_reporting",E_ALL); print_r
error_reporting(E_ALL);
$a=NULL;
if( isset($a))echo 'isset of variable $a is true';
echo '

isset situation:

';
if(isset($_GET['fo'])){
echo 'The isset of variable /'fo/' is true, the variable is available';
}else{
echo 'The isset of variable /'fo/' is false, no variable is set';
}
echo '

Empty situation:

';
if(empty($_GET['fo'])){
echo 'The empty value of variable /'fo/' is true, that is, an empty value or an invalid value';
} else{
echo 'The empty of variable /'fo/' is false and has a value';
}
echo '

is_numeric situation:

';
if(is_numeric($_GET['fo '])){ //When there is no fo parameter in the parameter, an error occurs.
echo 'The is_numeric of variable /'fo/' is true, it is a number';
}else{
echo 'The is_numeric of variable /'fo/' is false, it is not a number';
}
echo "

/ $_GET['fo']='' situation:

";
if($_GET['fo']==''){ //When there is no fo parameter in the parameter, an error occurs.
echo 'fo has no value, empty string';
}elseif($_GET['fo']!=''){
echo 'fo has a value, not /'/'.';
}
echo "

/$_GET['sex']='m':

";
if($_GET['sex']=='m'){ //When there is no parameter An error occurs when using the sex variable.
echo 'male';
}elseif($_GET['sex']=='f'){
echo 'female';
}
?>