How to sort a string by length of its words in php without using functions?

 $array = array('harish', 'mohan', 'jaideep', 'hari');

    for ($i = 1; $i < count($array); $i++) {
        for ($j = $i; $j > 0; $j--) {
            if (strlen($array[$j]) < strlen($array[$j - 1])) {

                $tmp = $array[$j];
                $array[$j] = $array[$j - 1];
                $array[$j - 1] = $tmp;
            }
        }
    }

    var_dump($array);

you could try this:

$string = "this is my test string";
$array = explode(" ", $string);
$result = array();
foreach ($array as $key => $string){
    $counter = 0;
    for($i = 0;;$i++){
        if (!isset($string[$i])){
            break;
        }
        $counter++;
    }
    $result[$counter][] = $string;
}

This splits your string and puts it into an array, where the keys are the counted characters. The problem now is, that you need to sort the array by the keys, which can be acquired by using ksort. I do not know if you may use it, if not, refer to this answer (use of sort) or this answer (no sort), this should do the trick (though I didn't test it).

This is the solution that I propose. I added some comments.

<?php

/* First part split the string in their words and store them in an array called $words. 
* The key is the length of the word and the value is an array with all the words having the same length as the key.
* e.g
* Array
(
    [4] => Array( [0] => Sort )
    [1] => Array( [0] => a )
    [6] => Array( [0] => string, [1] => length)
    [2] => Array( [0] => by, [1] => of)
    [3] => Array( [0] => its )
)
*/
$str = "Sort a string by length of its words";

$word = '';
$i = 0;
$word_length = 0;
$words = [];
$max = -1;

while( isset($str[$i]) ) {  
    if( $str[$i] !== ' ' ){
        $word .= $str[$i];
        $word_length++;     
    }else{
        //This is going to save the size of the longhest word in the array:     
        $max = ($word_length > $max) ? $word_length : $max; 
        //store the
        $words[$word_length][] = $word;                     
        $word = '';
        $word_length = 0;
    }
    $i++;
}

//print_r($words); // uncomment this if you wanna see content of the array.


//The if-condition is for ascending order or descending order.
$order = "DESC"; // "ASC" | "DESC"
$res = '';
if( $order === "DESC") {
    for( $i = $max; $i>=0 ; $i--) {
        if( ! isset($words[$i]) ) { continue; } 
        foreach($words[$i] as $word){
            $res .= $word . ' ';
        }               
    }
}else {
    //ascending order:
    for( $i = 0; $i<=$max; $i++) {
        if( ! isset($words[$i]) ) { continue; } 
        foreach($words[$i] as $word){
            $res .= $word . ' ';
        }               
    }
}

echo $res . "\n";
?>

Is this what you want?

Note: isset, echo, print, etc are PHP language constructs whereas print_r(), strlen(), etc. are built in functions. If you have doubts, you can see what's the difference in this post. What is the difference between a language construct and a "built-in" function in PHP?