Convert your numbers into strings like '10.3K' or '1M' for better readibility.

shortNumber.php

<?php

function shortNumber($number, $decimal = 2)
{
    $suffixes = [ 4 => '', 7 => 'K', 10 => 'M', 13 => 'B', 16 => 'T'];
    $absNumber = abs($number);
    $sign = ($number < 0) ? '-' : '';

    $magnitude = strlen(floor($absNumber)); // This is dirty, but it gets job done.

    $exponent = $magnitude % 3 === 0 ? $magnitude - 3 : $magnitude - ($magnitude % 3);

    if ($magnitude > 16) {
        $magnitude = 15;
        $exponent = 12;
    }

    $shortNumber = floor(pow(10, $decimal) * ($absNumber / pow(10, $exponent))) / pow(10, $decimal);

    foreach ($suffixes as $maxMagnitude => $suffix) {
        if ($magnitude < $maxMagnitude) {
            $shortNumber .= $suffix;
            break;
        }
    }
    
    return $sign . $shortNumber;
}

shortNumberTest.php

<?php

$testCases = [
    [ 0,                    2, '0' ],
    [ 999,                  2, '999' ],
    [ 1000,                 2, '1K' ],
    [ 1024,                 2, '1.02K' ],
    [ 99999,                2, '99.99K' ],
    [ 999999,               2, '999.99K' ],
    [ 1000000,              2, '1M' ],
    [ 1234567,              2, '1.23M' ],
    [ 99999999,             2, '99.99M' ],
    [ 999999999,            2, '999.99M' ],
    [ 1000000000,           2, '1B' ],
    [ 1234567890,           2, '1.23B' ],
    [ 99999999999,          2, '99.99B' ],
    [ 999999999999,         2, '999.99B' ],
    [ 1000000000000,        2, '1T' ],
    [ 1344843000000,        2, '1.34T' ],
    [ 9223372036854775807,  2, '9223372.03T' ],
];

echo '<pre>';
for ($i = 0; $i < count($testCases); $i++ ) {
    $arg1 = $testCases[$i][0];
    $arg2 = $testCases[$i][1];
    $expected = $testCases[$i][2];
    $result = shortNumber($arg1, $arg2);
    
    echo "($arg1,$arg2) => $result Expected: $expected" . PHP_EOL;
}
echo '</pre>';