Very easy my friend.
Say you have 50 gram or 50mL (both work) and the mass of phosphates in the solution is 0.05 grams (both have to be expressed as grams or mL ,not one milligrams and the other grams etc), so the ratio becomes:
Phosphate : Water
0.05 : 50
So you say okay, what if you i had million parts of water, how much phosphate will I have:
Phosphate : Water
0.05 : 50
x : 10^6
simple cross multiplication, gives you the value of x =( 0.05 * 10^6 )/50 = 1000 ppm.