If this cell is same as diagonally adjacent cell just above it , then same characters are present at str [ i - 1 ] and str [ j - 1 ] . Append any of them to result .
if ( $ dp [ $ i ] [ $ j ] == $ dp [ $ i - 1 ] [ $ j - 1 ] + 1 ) { $ res = $ res . $ str [ $ i - 1 ] ; $ i -- ; $ j -- ; }