Use round() (use if you are expecting a number in float format only, else use number_format() as an answer given by Codemwnci):

echo round(520.34345, 2);   // 520.34
echo round(520.3, 2);       // 520.3
echo round(520, 2);         // 520

From the manual:

Description:

float round(float precision = 0 [, int $mode = PHP_ROUND_HALF_UP ]]);

Returns the rounded value of val to specified precision (number of digits after the decimal point). precision can also be negative or zero (default).

...

Example #1 round() examples

<?php
    echo round(3.4);         // 3
    echo round(3.5);         // 4
    echo round(3.6);         // 4
    echo round(3.6, 0);      // 4
    echo round(1.95583, 2);  // 1.96
    echo round(1241757, -3); // 1242000
    echo round(5.045, 2);    // 5.05
    echo round(5.055, 2);    // 5.06
?>

###Example #2 mode examples

<?php
    echo round(9.5, 0, PHP_ROUND_HALF_UP);   // 10
    echo round(9.5, 0, PHP_ROUND_HALF_DOWN); // 9
    echo round(9.5, 0, PHP_ROUND_HALF_EVEN); // 10
    echo round(9.5, 0, PHP_ROUND_HALF_ODD);  // 9

    echo round(8.5, 0, PHP_ROUND_HALF_UP);   // 9
    echo round(8.5, 0, PHP_ROUND_HALF_DOWN); // 8
    echo round(8.5, 0, PHP_ROUND_HALF_EVEN); // 8
    echo round(8.5, 0, PHP_ROUND_HALF_ODD);  // 9
?>

Unfortunately, none of the previous answers (including the accepted one) works for all possible inputs.

1) sprintf('%1.'.$precision.'f', $val)

Fails with a precision of 2 : 14.239 should return 14.23 (but in this case returns 14.24).

2) floatval(substr($val, 0, strpos($val, '.') + $precision + 1))

Fails with a precision of 0 : 14 should return 14 (but in this case returns 1)

3) substr($val, 0, strrpos($val, '.', 0) + (1 + $precision))

Fails with a precision of 0 : -1 should return -1 (but in this case returns '-')

4) floor($val * pow(10, $precision)) / pow(10, $precision)

Although I used this one extensively, I recently discovered a flaw in it ; it fails for some values too. With a precision of 2 : 2.05 should return 2.05 (but in this case returns 2.04 !!)

So far the only way to pass all my tests is unfortunately to use string manipulation. My solution based on rationalboss one, is :

function floorDec($val, $precision = 2) {
    if ($precision < 0) { $precision = 0; }
    $numPointPosition = intval(strpos($val, '.'));
    if ($numPointPosition === 0) { //$val is an integer
        return $val;
    }
    return floatval(substr($val, 0, $numPointPosition + $precision + 1));
}

This function works with positive and negative numbers, as well as any precision needed.