How to use php function recursively and the difference between return and echo_PHP tutorial

//Simulate sql data
$array = array(0 =>'apple',1=>'banana',2=>'cat',3=>'dog',4=>'egg','5'=>'father');

//function usage 1
//arr is the incoming data $con is the condition
function f_1($arr,$con){
//array here is this function It is private inside and will not conflict with the outside array
//So, the outside array cannot be used directly inside, and the inside array cannot be used directly outside
//First instance an array
$ array = array();
//for foreach while usage is similar, specifically baidu
foreach ($arr as $key => $value) {
//If the value looped out is equal to con , add it to the array
if ($value == $con) {
//The difference between arrays and variables is the addition of []
$array[] = array($key => ; $value);
}
}
//Return the array after looping to get the result. Therefore, this function is an array
return $array;
//return will be terminated after execution, no matter what code follows it will not be executed
//return can be regarded as a function Ending place
}


//function usage 2
//$con can be an array
function f_2($arr,$con){
//First Instance a variable
$code = '

';
foreach ($arr as $key => $value) {
//The for loop inside is to loop out the con content
foreach ($con as $value2) {
// .= Add more continuous definition variables in the future
// If the value of the first layer of data loop is the same as the value of the second layer of conditional loop, Add to variables
//Multiple for loops to filter data are also called recursion
if ($value == $value2) {
$code .= '
• '.$value. '
';
}
}
}
$code .= '

';
//Return the variable after looping to get the result. So, this function is a string
return $code;
}

//function usage 3
//What is the difference between echo and return in the function? See the execution result
function f_3($arr,$con){
//First instance a variable
echo '

';
foreach ($arr as $key => $value) {
//The for loop inside is to loop out the con content
foreach ($con as $value2) {
// .= Add more continuously defined variables in the future
// If the first layer of data is looped out The value is the same as the value appearing in the second level conditional loop, and is added to the variable
//Multiple for loops to filter data are also called recursion
if ($value == $value2) {
echo '
• '.$value.'
';
}
}
}
echo '

';
}
?>

f_1 output start

//Because f_1 is an array, we can print it out
print_r(f_1($array,' banana'));
?>

f_1 output end

---

f_2 output start

//f_2 is a variable
$con = array('apple','father');
echo f_2($array,$con);
?>

f_2 output end

---

f_2 output start

// f_3 is already echoed in the function, so there is no need to echo when the function is executed
$con = array('apple','father');
f_3($array,$con);
?>

f_2 output end