<?php

function roundDown($decimal, $precision)
{
    $fraction = substr($decimal - floor($decimal), 2, $precision); // calculates the decimal places to $precision length
    $newDecimal = floor($decimal). '.' .$fraction; // reconstructs decimal with new decimal places
    
    return floatval($newDecimal);
}

echo roundDown(345.3467542, 5);
?>

345.34675