Converts an exponential (scientific) number notation to a decimal one using neither GMP nor BCMath module.

format-exponential.php

<?php
/**
 * Converts an exponential (scientific) number notation to a decimal one.
 * Uses neither GMP nor BCMath modules.
 *
 * @param float|int|string $value
 * @return string
 */
function formatExponential($value) {
	// Both upper-case “E” and lower-case “e” would work.
	$normalized  = strtolower(trim($value));

	$exponential = strpos($normalized, 'e');
	if($exponential === false) {
		if(strpos($normalized, '.') === false) {
			$integerPart = $normalized;
			$normalizedDecimalPart = '0';
		} else {
			list($integerPart, $decimalPart) = explode('.', $normalized);
			$normalizedDecimalPart = rtrim($decimalPart, '0');
		}

		$normalizedIntegerPart = ltrim($integerPart, '0');
		$formatted = $normalizedIntegerPart . '.' . $normalizedDecimalPart;
	} else {
		list($mantissa, $exponent) = explode('e', $normalized);

		// Removing + sign not to break figure count.
		$normalizedMantissa = ltrim($mantissa, '+');

		if(strpos($normalizedMantissa, '.') === false) {
			$mantissaIntegerPart = $normalizedMantissa;
			$flatMantissa        = $normalizedMantissa;
		} else {
			list($mantissaIntegerPart, $mantissaDecimalPart) = explode(
				'.', $normalizedMantissa
			);
			$flatMantissa = $mantissaIntegerPart . $mantissaDecimalPart;
		}

		$flatMantissaLength        = strlen($flatMantissa);
		$mantissaIntegerPartLength = strlen($mantissaIntegerPart);

		if($exponent >= 0) {
			$targetLength = $exponent + $mantissaIntegerPartLength;
			if($flatMantissaLength > $targetLength) {
				$intergerPart = substr($flatMantissa, 0, $targetLength);
				$decimalPart  = substr($flatMantissa, $targetLength);
				$formatted = $intergerPart . '.' . $decimalPart;
			} else { // $flatMantissaLength <= $targetLength
				$delta     = $targetLength - $flatMantissaLength;
				$formatted = $flatMantissa . str_repeat('0', $delta);
			}
		} else { // $exponent < 0
			$targetZeroesLength =
				abs($exponent) - $mantissaIntegerPartLength;
			$zeroes = str_repeat('0', $targetZeroesLength);
			$formatted = '0.' . $zeroes . $flatMantissa;
		}
	}

	return $formatted;
}

Hello. If you enter, for example, such a number -5.6258831787109375E-9, then it will not convert correctly.