Convert exponential number to decimal in php

You need a better math extension like BC Math, GMP... to handle the more precise precision.

Limitation of floating number & integer

You aren't doing anything wrong. Floats are notoriously innaccurate. From the docs (In the huge red warning box):

Floating point numbers have limited precision. Although it depends on the system, PHP typically uses the IEEE 754 double precision format, which will give a maximum relative error due to rounding in the order of 1.11e-16. Non elementary arithmetic operations may give larger errors, and, of course, error propagation must be considered when several operations are compounded.

Additionally, rational numbers that are exactly representable as floating point numbers in base 10, like 0.1 or 0.7, do not have an exact representation as floating point numbers in base 2, which is used internally, no matter the size of the mantissa. Hence, they cannot be converted into their internal binary counterparts without a small loss of precision. This can lead to confusing results: for example, floor((0.1+0.7)*10) will usually return 7 instead of the expected 8, since the internal representation will be something like 7.9999999999999991118....

So never trust floating number results to the last digit, and do not compare floating point numbers directly for equality. If higher precision is necessary, the arbitrary precision math functions and gmp functions are available.

Provide correct output for this so that we test and throw few more sample data.

Try this,

SELECT CONVERT(decimal(18,8), CAST('5E-05' AS FLOAT))