For cases such as : x , - x , + x
else if ( $ equation [ $ j ] == ' x ' ) { if ( ( $ i == $ j ) $ equation [ $ j - 1 ] == ' + ' ) $ coeff += $ sign ; else if ( $ equation [ $ j - 1 ] == ' - ' ) $ coeff -= $ sign ; else $ coeff += $ sign * intval ( substr ( $ equation , $ i , $ j - $ i ) ) ; $ i = $ j + 1 ; }