It took me some time to understand the way this function works…
I’ve compiled my own explanation with my own words that is more understandable for me personally than the official one or those that can be found in different tutorials on the web.
Perhaps, it will save someone several minutes…

<?php 
strspn(string $haystack, string $char_list [, int $start [, int $length]])
?>

The way it works:
 -   searches for a segment of $haystack that consists entirely from supplied through the second argument chars 
 -   $haystack must start from one of the chars supplied through $char_list, otherwise the function will find nothing
 -   as soon as the function encounters a char that was not mentioned in $chars it understands that the segment is over and stops (it doesn’t search for the second, third and so on segments)
 -   finally, it measures the segment’s length and return it (i.e. length)

In other words it finds a span (only the first one) in the string that consists entirely form chars supplied in $chars_list and returns its length

you can use this function with strlen to check illegal characters, string lenght must be the same than strspn (characters from my string contained in another)

<?php

$digits='0123456789';

if (strlen($phone) != strspn($phone,$digits))
 echo "illegal characters";

?>

This function is significantly faster for checking illegal characters than the equivalent preg_match() method.

very dificult to get from the definition directly, while i search for that,i came to know that

strspn() will tell you the length of a string consisting entirely of the set of characters in accept set. That is, it starts walking down str until it finds a character that is not in the set (that is, a character that is not to be accepted), and returns the length of the string so far. 

and 

strcspn() works much the same way, except that it walks down str until it finds a character in the reject set (that is, a character that is to be rejected.) It then returns the length of the string so far.

<?php
$acceptSet = "aeiou";
$rejectSet  = "y";

$str1 ="a banana";
$str2 ="the bolivian navy on manuvers in the south pacific";

echo $n = strspn($str1,$acceptSet);// $n == 1, just "a"

echo $n = strcspn($str2,$rejectSet);// n = 16, "the bolivian nav"
?>

hope this example will help in understanding the concept of strspn() and strcspn().

The second parameter is a set of allowed characters.
strspn will return an zero-based index of a first non-allowed character.

Quick way to check if a string consists entirely of characters within the mask is to compare strspn with strlen eg:

<?php
$path = $_SERVER['PATH_INFO'];
if (strspn($path,'/') == strlen($path)) {
    //PATH_INFO is empty
}
?>

Get Group match letter

<?php 

$s= 'aaabbbcceeffaaeeeaaabbzmmm';

function groupby( $s ){
    static $a = [];
    static $i = 0;
    
    $o = strspn( $s, $s[$i], $i);
    $a[ $i ] = [  $s[$i] => $o ];
    $i += $o;
    
    if( $i < strlen($s) ) {
        groupby($s);
    }

    return $a;
}

print_r(groupby($s));

?>

strspon and preg_match seem to be equally fast for validating numbers:

<?php

$testValInvalid = 'foobar123^^';
$testValValid = '12346';
$allowedChars = '1234567890';

$t1 = microtime(true);
for ($i = 0; $i < 1000000; $i++) {
    assert(strspn($testValInvalid, $allowedChars) != strlen($testValInvalid));
    assert(strspn($testValValid, $allowedChars) == strlen($testValValid));
}
print 'Time taken for strspon: ' . (microtime(true) - $t1);
print PHP_EOL;

$t1 = microtime(true);
for ($i = 0; $i < 1000000; $i++) {
    assert(preg_match('/^[0-9]+$/', $testValInvalid) === 0);
    assert(preg_match('/^[0-9]+$/', $testValValid));
}

print 'Time taken for preg_match: ' . (microtime(true) - $t1);
print PHP_EOL;

/**
nino-mcb:hosp_web ninoskopac$ php test.php
Time taken for strspon: 3.24165391922
Time taken for preg_match: 3.1820080280304
nino-mcb:hosp_web ninoskopac$ php test.php
Time taken for strspon: 3.1806418895721
Time taken for preg_match: 3.2244551181793
 */
?>