PHP substr return wrong value : Here is the example code :

    <?php
    
    $number = 0130;
    $a1     = substr($number,0,1);	
    $a2     = substr($number,1,1);
    $a3     = substr($number,2,1);
    $a4     = substr($number,3,1);
    $a      = [$a1,$a2,$a3,$a4];
    
    echo $a1;	

i got result is 8 why not 0?
